using System;

int sumOfNatural(int m, int n) {

    if (m <= 0 || n <= 0)

        throw new ArgumentException("Both numbers should be natural numbers");

    else if (m > n)

        throw new ArgumentException("The second number should be greater than the first one");

    if (m == n)

        return m;

    return n + sumOfNatural(m, n - 1);

}

​

try {

    Console.WriteLine("Enter the first natural number:");

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

    Console.WriteLine("Enter the second natural number greater than the first one:");

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

    Console.WriteLine(sumOfNatural(m, n));

} catch (ArgumentException e) {

    Console.WriteLine(e.Message);

}

​