using System;

using System.Collections.Generic;

public class Program

{

    class Solution 

    { 

        public int solution(int X, int[] A)

        {

            var sum = X * (X+1) / 2;

            var c = new HashSet<int>();

            for (var i = 1; i <= X; i++)

            {

                c.Add(i);

            }

            for (var i = 0; i < A.Length; i++)

            {

                if (A[i] > X)

                {

                    continue;

                }

                if (c.Contains((A[i])))

                {

                    sum -= A[i];

                    c.Remove(A[i]);

                }

                if (sum == 0)

                {

                    return i;   

                }

            }

            return -1;

        }

    }

    public static void Main()

    {

        var s = new Solution();

        Console.WriteLine(s.solution(5,new int[]{1, 3, 1, 4, 2, 3, 5}));

    }

}