extras: add P16-P23, P25

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

P19.java

@@ -0,0 +1,17 @@
+import java.util.Scanner;
+
+public class P19 {
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+
+        System.out.print("Enter the first number: ");
+        int a = scanner.nextInt();
+
+        System.out.print("Enter the second number: ");
+        int b = scanner.nextInt();
+
+        System.out.println("Sum: " + (a + b));
+
+        scanner.close();
+    }
+}