using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace No143
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] inputs = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int sum_oldyars = Console.ReadLine().Split().Select(int.Parse).Sum();

            int result = inputs[0] * inputs[1] - sum_oldyars;
            
            Console.WriteLine(result>=0 ? result : -1);
        }
    }
}