String buider . Append

I am not getting the output and i am getting the arugument out of range exception for the same. 

 

//Write a program that reads a string from the console (20 characters maximum) and if shorter complements it right with "*" to 20 characters.

StringBuilder first = new StringBuilder(50);

string input = Console.ReadLine();

if (input.Length<=20)

{

first.Append(input);

Console.WriteLine(first);

first.Append("x", (input.Length + 1), (20 - input.Length-1));

}

Console.WriteLine(first);

 

 

My input is : Harish

Expected output is : HARISHXXXXXXXXXXXXXX