using System.Linq;
using System;

public class Paiza
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var k = int.Parse(line[0]);
        var n = int.Parse(line[1]);
        line = Console.ReadLine().Trim().Split(' ');
        var a = Array.ConvertAll(line, int.Parse);
        var w = k * n - a.Sum();
        Console.WriteLine(w >= 0 ? w : -1);
    }
}