using System;
using System.Linq;

class Program
{
    public void Solve()
    {
        int[] KNF = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        int[] A = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();

        int bean = KNF[0] * KNF[1];
        int ageSum = A.Sum();

        if (bean - ageSum < 0)
        {
            Console.WriteLine(-1);
        }
        else
        {
            Console.WriteLine(bean - ageSum);
        }
    }

    static void Main()
    {
        var solver = new Program();
        solver.Solve();
    }
}