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

class Program {
    static List<int> GetNumberListFromString(string input, char delimitor = ' ') {
        return input.Split( delimitor ).ToList<string>().ConvertAll<int>( int.Parse );
    }
    static void Main() {
        List<int> knf = GetNumberListFromString( Console.ReadLine() );
        List<int> ages = GetNumberListFromString( Console.ReadLine() );
        int requiredBeans = ages.Sum();
        Console.WriteLine( knf[ 0 ] * knf[ 1 ] >= requiredBeans ? knf[ 0 ] * knf[ 1 ] - requiredBeans : -1 );
    }
}