using System;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        int[] arr1 = Console.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();
        int[] arr2 = Console.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();

        int exist = arr1[0] * arr1[1];
        int need = arr2.Sum();
        int ans;

        if (exist - need >= 0)
        {
            ans = exist - need;
        }
        else
        {
            ans = -1;
        }

        Console.WriteLine(ans);
        Console.Read();
    }
}