using System;
using System.Linq;

namespace TestConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            var firstInput = Console.ReadLine().Split();
            var secoundinput = Console.ReadLine().Split();

            int beans = int.Parse(firstInput[0]) * int.Parse(firstInput[1]);
            int ageTotal = secoundinput.Select(i => int.Parse(i)).Sum();

            if (beans < ageTotal)
            {
                Console.WriteLine(-1);
            }
            else
            {
                Console.WriteLine(beans - ageTotal);
            }
        }
    }
}