Find Max Number Of Occurence in the int array, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] sample = { 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 7, 8, 9, 11, 12, 13, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2 };
            Console.WriteLine("Max = {0}", FindMaxNumberOfOccurence(sample));
        }
        public static int FindMaxNumberOfOccurence(int[] sample)
        {
            Dictionary<int, int> dict = new Dictionary<int, int>();
            foreach (var item in sample)
            {
                if (!dict.Keys.Contains(item))
                {
                
                dict.Add(item, 1);
                }
                else
                dict[item] += 1;
            }
            return dict.FirstOrDefault(x => x.Value == dict.Values.Max()).Key;
        }        
   }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,08 sec, cpu time: 0,08 sec, average memory usage: 13 Mb, average nr of threads: 3