using System;
using System.Linq;

public class Hello{
    public static void Main(){
        int[] input1 = Console.ReadLine().Trim().Split(' ').Select(x => int.Parse(x)).ToArray();
        int[] input2 = Console.ReadLine().Trim().Split(' ').Select(x => int.Parse(x)).ToArray();

        int beans = input1[0] * input1[1];
        for(int i = 0; i < input2.Length; i++)
        {
            beans = beans - input2[i];
        }
        
        if(beans < 0)
        {
            beans = -1;
        }
        
        Console.WriteLine(beans < 0 ? -1 : beans);
        
    }
}