Find the duplicate element in an array with O(1) space complexity

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

namespace OneCompiler
{
	public class Program
	{
		public static void Main(string[] args)
		{
			int n = int.Parse(Console.ReadLine());
			
			int[] arr = Array.ConvertAll(Console.ReadLine().Split(" "), new Converter<string,int>(StringToInteger));
			
			var duplicate = Duplicate(arr);
			
			Console.WriteLine(duplicate);
		}
		
		private static int StringToInteger(string s)
		{
		  return int.Parse(s);
		}
		
		private static int Duplicate(int[] arr)
		{
		  int n = arr.Length;
		  
		  int duplicate = -1;
		  
		  for (int i = 0; i < n; i++)
		  {
		    arr[arr[i] % n] = arr[arr[i] % n] + n;
		  }
		  
		  for (int i = 0; i < n; i++)
		  {
		    if ((arr[i] / n) > 1)
		    {
		      duplicate = i;
		      break;
		    }
		  }
		  
		  return duplicate;
		}
	}
}