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

namespace YukiCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var va = Console.ReadLine().Split().Select(int.Parse).ToList();
            int sn = Console.ReadLine().Split().Select(int.Parse).Sum();
            int rn = va[0] * va[1] - sn;
            if (rn < 0) rn = -1;

            Console.WriteLine(rn);
            Console.ReadLine();
        }
    }
}