using System;

public class Program

{

    public static void Main()

    {

        int num1, num2, count,sum;

        Console.WriteLine("enter natural number");

        num1 = int.Parse(Console.ReadLine());

        Console.WriteLine("enter natural number");

        num2 = int.Parse(Console.ReadLine());

        int max = Math.Max(num1,num2);

        int min = Math.Min(num1,num2);

        sum=0;

        for (count=min;count<=max;count++)

            sum = sum+count;

        Console.WriteLine(sum);

    }

}