feat: implement sl_3 and sl_4 solutions

sl_3/AddNumbers.java

@@ -0,0 +1,12 @@
+import java.util.Scanner;
+
+public class AddNumbers {
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        System.out.print("Enter first number: ");
+        int a = sc.nextInt();
+        System.out.print("Enter second number: ");
+        int b = sc.nextInt();
+        System.out.println("Sum: " + (a + b));
+    }
+}

sl_4/LeapYear.java

@@ -0,0 +1,15 @@
+import java.util.Scanner;
+
+public class LeapYear {
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        System.out.print("Enter a year: ");
+        int year = sc.nextInt();
+        boolean isLeap = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
+        if (isLeap) {
+            System.out.println(year + " is a leap year.");
+        } else {
+            System.out.println(year + " is not a leap year.");
+        }
+    }
+}