using System;

public class Program {
	public static void Main(string[] args) {
		string[] s = Console.ReadLine().Split(' ');
		string[] ages = Console.ReadLine().Split(' ');
		int beans = int.Parse(s[0]) * int.Parse(s[1]);
		for (int i = 0; i < ages.Length; i++) {
			beans -= int.Parse(ages[i]);
		}
		if (beans >= 0) {
			Console.WriteLine(beans);
		} else {
			Console.WriteLine("-1");
		}
	}
}