using System;


namespace ConsoleApplication5
{
    class Program
    {
        static void Main(string[] args)
        {
            int i,s=0;
            string[] A = Console.ReadLine().Split(' ');
            string[] B = Console.ReadLine().Split(' ');
            int min = int.Parse(A[1]);
            int car = int.Parse(A[0]);
            for (i = 0; i < min; i++)
            {

                s = s + int.Parse(B[i]);
            } int a=s-(min*car);
            if (a >= 0)
                Console.WriteLine(a);
            else
                Console.WriteLine(0);

        }
    }
}