using System;

public class Program

{

    public static void Main()

    {

        int bulanan, guna1, total = 0, hitung = 0;

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

        guna1 = int.Parse(Console.ReadLine());

        int i;

        for (i = 0; i < guna1; i++) {

            int nguna;

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

            total += nguna;

        }

        hitung = (guna1 + 1) * bulanan - total;

        Console.WriteLine(hitung);

    }

}