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

class Program
{
    static string InputPattern = "Input2";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("10 10 3");
            WillReturn.Add("10 20 40");
            //30
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();
        int[] wkArr = InputList[0].Split(' ').Select(X => int.Parse(X)).ToArray();
        int K = wkArr[0];
        int N = wkArr[1];
        int[] AArr = InputList[1].Split(' ').Select(X => int.Parse(X)).ToArray();

        int RestCnt = K * N - AArr.Sum();
        Console.WriteLine(RestCnt >= 0 ? RestCnt : -1);
    }
}