What is the statement or expression at ?4?

boolean Palindrome(int N)
{
    int rev = ?1?;
    int num = N;
    while (num > 0)
    {
        int f = num / 10;
        int s = ?2?;
        int digit = num - ?3?;
        rev = ?4? + digit;
        num /= ?5?;
    }
    if(rev == N)
        return true;
    else
        return false;
}