I have added my factorial code in C language

C/factorial.c

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+#include <stdio.h>
+int main() {
+    int n, i;
+    unsigned long long fact = 1;
+    printf("Enter an integer: ");
+    scanf("%d", &n);
+
+    // shows error if the user enters a negative integer
+    if (n < 0)
+        printf("Error! Factorial of a negative number doesn't exist.");
+    else {
+        for (i = 1; i <= n; ++i) {
+            fact *= i;
+        }
+        printf("Factorial of %d = %llu", n, fact);
+    }
+
+    return 0;
+}

C/readme.md

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+# Factorial
+
+
+
+In mathematics, the factorial of a non-negative integer `n`, 
+denoted by `n!`, is the product of all positive integers less 
+than or equal to `n`. For example:
+
+```
+5! = 5 * 4 * 3 * 2 * 1 = 120
+```
+
+| n     | n!                          | 
+| ----- | --------------------------: |
+| 0     | 1                           |
+| 1     | 1                           |
+| 2     | 2                           |
+| 3     | 6                           |
+| 4     | 24                          |
+| 5     | 120                         |
+| 6     | 720                         |
+| 7     | 5 040                       |
+| 8     | 40 320                      |
+| 9     | 362 880                     |
+| 10    | 3 628 800                   |
+| 11    | 39 916 800                  |
+| 12    | 479 001 600                 |
+| 13    | 6 227 020 800               |
+| 14    | 87 178 291 200              |
+| 15    | 1 307 674 368 000           |
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Factorial)