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

namespace test20190423
{
    class Program
    {
        static void Main(string[] args)
        {
            var sum = Console.ReadLine().Split(new String[] { " " }, StringSplitOptions.RemoveEmptyEntries).Take(2).Select(x => int.Parse(x)).Aggregate((x, y) => x * y);
            var age = Console.ReadLine().Split(new String[] { " " }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).Sum();
            Console.WriteLine(sum >= age ? sum - age : -1);
            Console.ReadLine();
        }
    }
}