using System;
using System.Linq;

class Sample
{
    public static void Main()
    {
        var beansList = Console.ReadLine().Trim().Split(" ")
            .Select(int.Parse).ToList();
        var ages = Console.ReadLine().Trim().Split(" ")
            .Select(int.Parse).ToList();
        var beans = 0;
        var sum = 0;
        for(int i = 0; i < ages.Count(); i++)
        {
            sum += ages[i];
        }
        beans = beansList[0] * beansList[1];
        if (beans < sum)
        {
            Console.WriteLine(-1);
        }
        else
        {
            Console.WriteLine(beans - sum);
        }
       
    }
}