using System;
using System.Linq;
public class Program {
  public static void Main() {
    int[] KNF = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
    int[] A = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();

    int remain = KNF[0] * KNF[1] - A.Sum();

    if (remain >= 0) {
      Console.WriteLine(remain);
    } else {
      Console.WriteLine(-1);
    }
  }
}