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

public class Test
{
	public static void Main()
	{
		var m = Console.ReadLine()
			.Split(' ')
			.Select(x => int.Parse(x))
			.ToList();
		var n = Console.ReadLine()
			.Split(' ')
			.Select(x => int.Parse(x))
			.ToList();

		var ans = (m[0] * m[1]) >= n.Sum() ? m[0] * m[1] - n.Sum() : -1;
		
		Console.WriteLine(ans);
	}
}