using System;
using System.Linq;

public class Program
{
	static void Main()
	{
		var BeanValue = Console.ReadLine().Split(' ').Select(x => Convert.ToInt32(x)).ToList<int>();
		var FamilyValue = Console.ReadLine().Split(' ').Select(x => Convert.ToInt32(x)).ToList<int>();

		var Beans = BeanValue[0] * BeanValue[1];

		foreach (var Family in FamilyValue)
			Beans -= Family;

		if (Beans < 0)
			Console.WriteLine("-1");
		else
			Console.WriteLine(Beans);
	}
}