using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static int FindSpecialNumber(int[] arr)

    {

        int len = arr.Length;        

        if (len == 1)

            return arr[0] == 1 ? 1 : -1;

        var sortedMap = new SortedDictionary<int, int>();

        for (int i = 0; i < len; i++)

        {

            if (!sortedMap.ContainsKey(arr[i]))

                sortedMap[arr[i]] = 0;

            sortedMap[arr[i]]++;

        }

        for (int i = sortedMap.Count - 1; i >= 0; i--)

        {

            var item = sortedMap.ElementAt(i);

            if (item.Key == item.Value)

                return item.Key;

        }

        return -1;

    }

    public static void Main()

    {

        Console.WriteLine("UniLecs");

        // tests

        Console.WriteLine(FindSpecialNumber(new int[] { 2, 2, 3, 4 }).ToString()); // 2

        Console.WriteLine(FindSpecialNumber(new int[] { 1, 2, 2, 3, 3, 3 }).ToString()); // 3

        Console.WriteLine(FindSpecialNumber(new int[] { 2, 2, 2, 3, 3 }).ToString()); // -1

    }

}