using System;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        string s1 = Console.ReadLine();
        string s2 = Console.ReadLine();
        int output;

        int[] array1 = s1.Split(' ').Select(a => int.Parse(a)).ToArray();
        int[] array2 = s2.Split(' ').Select(a => int.Parse(a)).ToArray();

        int exist = array1[0] * array1[1];
        int need = array2.Sum();

        if (exist > need)
        {
            output = exist - need;
        }
        else
        {
            output = -1;
        }

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