using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {

        int[] KNF = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        int[] A = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();

        Console.WriteLine(KNF[0] * KNF[1] - A.Sum() < 0 ? -1 : KNF[0] * KNF[1] - A.Sum());
    }
}