using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ViragosRetRevised
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] inp = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int n = inp[0];
            int f = inp[1];
            inp = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int g = inp.Sum();
            int sol = 0;
            for (int i = 1; i <= g - f + 1; i++)
            {
                sol += i;
            }
            Console.WriteLine(sol);
            Console.ReadLine();
        }
    }
}