using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using static System.Console;

class Program
{
    static void Main()
    {
        var knf = ReadLine().Split().Select(int.Parse).ToArray();
        var a = knf[0] * knf[1] - ReadLine().Split().Select(int.Parse).ToList().Sum();
        WriteLine(a > -1 ? a : -1);
    }
}