diff --git a/src/Source.cpp b/src/Source.cpp
new file mode 100644
index 0000000..ca779dc
--- /dev/null
+++ b/src/Source.cpp
@@ -0,0 +1,383 @@
+#include <iostream>
+#include <vector>
+#include <string>
+#include <stack>
+#include <set>
+using namespace std;
+
+//оператор ^ не работает корректно не учтена правая ассоциативность
+//для корректной работы используйте скобки: 2^(3^2) вместо 2^3^2
+enum class TypeLexeme
+{
+	number,
+	variable,
+	un_op,
+	bin_op,
+	left_bracket,
+	right_bracket,
+	error
+};
+
+enum class Priority
+{
+	number,
+	bracket,
+	low,
+	mid,
+	high
+};
+
+struct Lexeme
+{
+	TypeLexeme type;
+	std::string value;
+	Priority priority;
+};
+
+enum class LexemeState
+{
+	start,
+	number,
+	variable,
+	un_op,
+	bin_op,
+	left_bracket,
+	right_bracket,
+	end,
+	error
+};
+bool check_arithmetic_expression(const std::string& s);
+Lexeme convertToLexeme(const std::string &expression, size_t &index, LexemeState state)
+{
+	std::string value;
+	if (expression[index] >= '0' && expression[index] <= '9')
+	{
+		if (expression[index + 1] == '0')
+		{
+			value.push_back('0');
+			++index;
+			return Lexeme({ TypeLexeme::number, value, Priority::number });
+		}
+		while (expression[index] >= '0' && expression[index] <= '9')
+		{
+			value.push_back(expression[index]);
+			++index;
+		}
+		return Lexeme({ TypeLexeme::number, value, Priority::number });
+	}
+	else if (expression[index] == '*' || expression[index] == '/')
+	{
+		value.push_back(expression[index]);
+		index++;
+		return Lexeme({ TypeLexeme::bin_op, value, Priority::mid });
+	}
+	else if (expression[index] == '^')
+	{
+		value.push_back(expression[index]);
+		index++;
+		return Lexeme({ TypeLexeme::bin_op, value, Priority::high });
+	}
+	else if (expression[index] == '+' || (expression[index] == '-' && (state == LexemeState::variable || state == LexemeState::number || state == LexemeState::right_bracket)))
+	{
+		value.push_back(expression[index]);
+		index++;
+		return Lexeme({ TypeLexeme::bin_op, value, Priority::low });
+	}
+	else if (expression[index] == '-' && (state == LexemeState::left_bracket || state == LexemeState::start))
+	{
+		value.push_back(expression[index]);
+		index++;
+		return Lexeme({ TypeLexeme::un_op, value, Priority::high });
+	}
+	else if (expression[index] == '(')
+	{
+		value.push_back(expression[index]);
+		index++;
+		return Lexeme({ TypeLexeme::left_bracket, value, Priority::bracket });
+	}
+	else if (expression[index] == ')')
+	{
+		value.push_back(expression[index]);
+		index++;
+		return Lexeme({ TypeLexeme::right_bracket, value, Priority::bracket });
+	}
+	else if (expression.substr(index, 3) == "sin" || expression.substr(index, 3) == "cos")
+	{
+		if (expression.substr(index, 3) == "sin")
+		{
+			value = "sin";
+			index += 3;
+			return Lexeme({ TypeLexeme::un_op, value, Priority::high });
+		}
+		else if (expression.substr(index, 3) == "cos")
+		{
+			value = "cos";
+			index += 3;
+			return Lexeme({ TypeLexeme::un_op, value, Priority::high });
+		}
+	}
+	else if (expression[index] >= 'a' && expression[index] <= 'z')
+	{
+		value.push_back(expression[index]);
+		index++;
+		return Lexeme({ TypeLexeme::variable, value, Priority::low });
+	}
+	return Lexeme({ TypeLexeme::error, value, Priority::low });
+}
+
+std::vector<Lexeme> convertStrToLexemes(const std::string& expression)
+{
+	int scopeCounter = 0;
+	std::vector<Lexeme> res;
+	LexemeState state = LexemeState::start;
+	size_t index = 0;
+	while (index < expression.size())
+	{
+		const Lexeme lexeme = convertToLexeme(expression, index, state);
+		if (state == LexemeState::start)
+		{
+			if (lexeme.type == TypeLexeme::un_op)
+			{
+				state = LexemeState::un_op;
+			}
+			else if (lexeme.type == TypeLexeme::variable || lexeme.type == TypeLexeme::number)
+			{
+				state = LexemeState::variable;
+			}
+			else if (lexeme.type == TypeLexeme::left_bracket)
+			{
+				state = LexemeState::left_bracket;
+				scopeCounter++;
+			}
+			else throw "Error: invalid expression format (start lexeme is incorrect)";
+		}
+		else if (state == LexemeState::un_op)
+		{
+			if (lexeme.type == TypeLexeme::variable || lexeme.type == TypeLexeme::number)
+			{
+				state = LexemeState::variable;
+			}
+			else if (lexeme.type == TypeLexeme::un_op)
+			{
+				state = LexemeState::un_op;
+			}
+			else if (lexeme.type == TypeLexeme::left_bracket)
+			{
+				state = LexemeState::left_bracket;
+				scopeCounter++;
+			}
+			else throw "Error: invalid expression after unary operator";
+		}
+		else if (state == LexemeState::variable)
+		{
+			if (lexeme.type == TypeLexeme::bin_op)
+			{
+				state = LexemeState::bin_op;
+			}
+			else if (lexeme.type == TypeLexeme::right_bracket)
+			{
+				state = LexemeState::right_bracket;
+				scopeCounter--;
+			}
+			else throw "Error: invalid expression after a variable";
+		}
+		else if (state == LexemeState::bin_op)
+		{
+			if (lexeme.type == TypeLexeme::variable || lexeme.type == TypeLexeme::number)
+			{
+				state = LexemeState::variable;
+			}
+			else if (lexeme.type == TypeLexeme::left_bracket)
+			{
+				state = LexemeState::left_bracket;
+				scopeCounter++;
+			}
+			else if (lexeme.type == TypeLexeme::un_op)
+			{
+				state = LexemeState::un_op;
+			}
+			else throw "Error: invalid expression after a binary operator";
+		}
+		else if (state == LexemeState::left_bracket)
+		{
+			if (lexeme.type == TypeLexeme::left_bracket)
+			{
+				state = LexemeState::left_bracket;
+				scopeCounter++;
+			}
+			else if (lexeme.type == TypeLexeme::un_op)
+			{
+				state = LexemeState::un_op;
+			}
+			else if (lexeme.type == TypeLexeme::variable || lexeme.type == TypeLexeme::number)
+			{
+				state = LexemeState::variable;
+			}
+			else if (lexeme.type == TypeLexeme::un_op)
+			{
+				state = LexemeState::un_op;
+			}
+			else throw "Error: invalid expression after a left bracket";
+		}
+		else if (state == LexemeState::right_bracket)
+		{
+			if (lexeme.type == TypeLexeme::right_bracket)
+			{
+				state = LexemeState::right_bracket;
+				scopeCounter--;
+			}
+			else if (lexeme.type == TypeLexeme::bin_op)
+			{
+				state = LexemeState::bin_op;
+			}
+			else throw "Error: invalid expression after a right bracket";
+		}
+		else if (scopeCounter < 0) throw "Error: brackets are out of order";
+		else throw "error";
+		res.push_back(lexeme);
+	}
+	return res;
+}
+
+std::vector<Lexeme> reverse_Polish_notation(const std::vector<Lexeme> &src)
+{
+	std::vector<Lexeme> out;
+	std::stack<Lexeme> op_stack;
+	for (const Lexeme &lex : src)
+	{
+		if (lex.type == TypeLexeme::variable || lex.type == TypeLexeme::number)
+		{
+			out.push_back(lex);
+		}
+		else if (lex.type == TypeLexeme::left_bracket || lex.type == TypeLexeme::un_op)
+		{
+			op_stack.push(lex);
+		}
+		else if (lex.type == TypeLexeme::right_bracket)
+		{
+			while (!op_stack.empty() && (op_stack.top().type != TypeLexeme::left_bracket))
+			{
+				out.push_back(op_stack.top());
+				op_stack.pop();
+			}
+			op_stack.pop();
+		}
+		else if (lex.type == TypeLexeme::bin_op)
+		{
+			while (!op_stack.empty() && (op_stack.top().type == TypeLexeme::un_op || op_stack.top().priority >= lex.priority))
+			{
+				out.push_back(op_stack.top());
+				op_stack.pop();
+			}
+			op_stack.push(lex);
+		}
+	}
+	while (!op_stack.empty())
+	{
+		out.push_back(op_stack.top());
+		op_stack.pop();
+	}
+	return out;
+}
+
+double calc_rpn(std::vector<Lexeme> &src_rpn)
+{
+	std::stack<double> tmp_stack;
+	for (size_t i = 0; i < src_rpn.size(); ++i)
+	{
+		if (src_rpn[i].type == TypeLexeme::un_op)
+		{
+			if (src_rpn[i].value == "-")
+			{
+				double operand = tmp_stack.top();
+				tmp_stack.pop();
+				tmp_stack.push(-operand);
+			}
+			else if (src_rpn[i].value == "sin")
+			{
+				double operand = tmp_stack.top();
+				tmp_stack.pop();
+				tmp_stack.push(sin(operand));
+			}
+			else if (src_rpn[i].value == "cos")
+			{
+				double operand = tmp_stack.top();
+				tmp_stack.pop();
+				tmp_stack.push(cos(operand));
+			}
+		}
+		else if (src_rpn[i].type == TypeLexeme::number || src_rpn[i].type == TypeLexeme::variable)
+		{
+			tmp_stack.push(stoi(src_rpn[i].value));
+		}
+		else if (src_rpn[i].type == TypeLexeme::bin_op)
+		{
+			double second_operand = tmp_stack.top();
+			tmp_stack.pop();
+			double first_operand = tmp_stack.top();
+			tmp_stack.pop();
+			if (src_rpn[i].value == "+")
+			{
+				tmp_stack.push(first_operand + second_operand);
+			}
+			else if (src_rpn[i].value == "-")
+			{
+				tmp_stack.push(first_operand - second_operand);
+			}
+			else if (src_rpn[i].value == "*")
+			{
+				tmp_stack.push(first_operand * second_operand);
+			}
+			else if (src_rpn[i].value == "^")
+			{
+				tmp_stack.push(std::pow(first_operand, second_operand));
+			}
+			else if (src_rpn[i].value == "/")
+			{
+				if (second_operand == 0) throw "Division by zero";
+				tmp_stack.push(first_operand / second_operand);
+			}
+		}
+	}
+	return tmp_stack.top();
+}
+
+int main()
+{
+	std::string expression("-3+19^5*(-sin(x))-(-3)+18*cos(y)");
+	std::vector<Lexeme> lexems = convertStrToLexemes(expression);
+	std::set<char> variables;
+	for (size_t i = 0; i < lexems.size(); ++i)
+	{
+		if (lexems[i].type == TypeLexeme::variable)
+		{
+			variables.insert(lexems[i].value[0]);
+		}
+	}
+	std::set<char>::iterator it = variables.begin();
+	while (it != variables.end())
+	{
+		double variable_value;
+		std::cout << "Enter the value of " << *it << ": " << std::endl;
+		std::cin >> variable_value;
+		for (size_t j = 0; j < lexems.size(); ++j)
+		{
+			if (lexems[j].value[0] == *it)
+			{
+				lexems[j].value = std::to_string(variable_value);
+				lexems[j].type = TypeLexeme::number;
+			}
+		}
+		it++;
+	}
+	std::vector<Lexeme> RPN = reverse_Polish_notation(lexems);
+	std::cout << calc_rpn(RPN);
+	system("pause");
+	return 0;
+}
+
+// API
+bool check_arithmetic_expression(const std::string& s)
+{
+	return false;
+}