using System;
using System.Globalization;
using System.Linq;
using System.Collections.Generic;
 
public class Test
{
 
 
	public static void Main()
	{
        string input = ":1Af404080A83hfndsgt4u47";
        string replace = "Hello123";
        int start = 5;
        string result = "";
        if (input.Length >= start)
        {
            result = input.Substring(0, start) + replace + input.Substring(start + replace.Length);
            Console.Write(result);
        }
	}
}

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