using System;
using System.Numerics;
public class Program
{
    static void Main()
    {
        var knf = ReadLineLong();
        var a = ReadLineLong();
        var b = knf[0] * knf[1];
        var s = a.Sum();
        Console.WriteLine(b - s < 0 ? -1 : b - s);
    }

    static string[] ReadLineSplit()
    {
        var s = Console.ReadLine() ?? "";
        return s.Split(' ');
    }
    static long[] ReadLineLong()
    {
        var s = Console.ReadLine() ?? "";
        return s.Split(' ').Select(long.Parse).ToArray();
    }
}