Miért nem jó ez a c# kód?

namespace Practice

{

class MainClass

{

public static void Main (string[] args)

{

const byte targetNumber = 5;

byte counter = 0;

long num = 0;

bool isAgain = false;

do {

Console.Write ("Enter a number: ");

num = Convert.ToInt64 (Console.ReadLine ());

string strnum = num.ToString();

char chtargetnum = Convert.ToChar (targetNumber);

for (byte i = (byte)(Math.Log10(num) + 1); i > 0; --i) {

if (strnum [i-1] == chtargetnum)

++counter;

}

Console.Write ("There are a total piece of " + counter + "\nCan you play again(y,n): ");

char again = Console.ReadLine () [0];

if (again == 'y') {

isAgain = true;

counter = 0;

}

else

isAgain = false;

} while(isAgain);

Console.ReadKey ();

}

}

}