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

namespace yukicoder_143 {
    class Program {
        static void Main(string[] args) {
            string[] s = Console.ReadLine().Split(' ');
            string[] t = Console.ReadLine().Split(' ');
            int[] p = new int[s.Length];
            int[] q = new int[t.Length];
            int sum = 0;
            for(int i = 0; i < s.Length; i++) {
                p[i] = int.Parse(s[i]);
            }
            for(int i = 0; i < t.Length; i++) {
                q[i] = int.Parse(t[i]);
                sum += q[i];
            }
            if (p[0] * p[1] - sum < 0) {
                Console.WriteLine(-1);
            } else {
                Console.WriteLine(p[0] * p[1] - sum);
            }
            Console.ReadLine();

        }
    }
}