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

namespace PracticeAtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
			
			
			//中央値
			string sA = Console.ReadLine();
			string sB = Console.ReadLine();
			string[] tA = sA.Split(' ');
			string[] tB = sB.Split(' ');
			Boolean flg = true;
			int sumA = 0 ;
			sumA = int.Parse(tA[0]) * int.Parse(tA[1]);
			int sumB = 0 ;
			//sumA = tA[0] * tA[1];
			//　年齢を合計する　Q143　豆
			foreach(var a in tB){
				sumB = sumB +int.Parse(a);
			}
			if(sumA >= sumB){
				Console.WriteLine(sumA - sumB);
				
			}else{
				Console.WriteLine(-1);
				
			
			}
		}
    }
}