using System;

using System.Text;

public class Program

{

​

    public static void Main()

    {

        int numeroDeLetras = 5;

        StringBuilder telaBuilder = new StringBuilder(new string('-', numeroDeLetras));

​

        Console.WriteLine(telaBuilder.ToString());

        telaBuilder[3] = 'C';

        Console.WriteLine(telaBuilder.ToString());

    }

}