using System;
using System.Text;
 
namespace Program
{
        class ProgramOne
        {
        	public static void Main()
        	{
                	string str = "              aa bb",s;
                	
                	foreach(char c in str)
			{
				Console.WriteLine(c);
			}
		
			for(int i=0;i<str.Length;i++)
			{
					if(str[i].Equals('a'))
					{	
						Console.WriteLine("Replacing");	
						str=str.Replace("a","xxxx");
						
					}
					
			Console.WriteLine(str);			
						
			}
	
				
			
        	}
	}
}