using System;

public class Program

{

    public static void Main()

    {

        string [] V = Console.ReadLine().Split();

        int A = int.Parse(V[0]), N = 0;

        for (int i = 1; i < V.Length; i ++) {

            if (int.Parse(V[i]) > 0 && N == 0)

                N = int.Parse(V[i]);

        }

        int S = A;

        for (int j = 1; j < N; j ++) {

            S += A + j;

        }

        Console.WriteLine(S);

  }

}