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

namespace template
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int[] y = Console.ReadLine().Split().Select(int.Parse).ToArray();
            Console.WriteLine(x[0] * x[1] < y.Sum() ? -1 : x[0] * x[1] - y.Sum());
        }
    }
}