using System;
using System.Linq;

public class Program
{
    public static void Main()
    {
        var l = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToList();
        var values = Console.ReadLine().Split(' ');

        var total = l[0] * l[1];
        var amount = values.Sum(i => int.Parse(i));

        Console.Write("{0}", (total >= amount) ? (total - amount) : -1);

        return;
    }
}