using System;
using System.Linq;

public class P0143 {
    public static void Main() {
        var knf = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToList();
        var a = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToList();
        var beans = knf[0] * knf[1];
        var ages = a.Sum();
        Console.WriteLine(beans < ages ? -1 : beans - ages);
    }
}