In this program there are two different errors: failure to declare the
varialbe N and failure to anticipate that N could be zero at the time
line 13 is reached during execution.  The first error is a compile-time
error, due to the fact that it can be detected at compile time rather
than execution time.  The second error is an execution-time error because
it cannot be detected at compile time; the value of N is not known until
after execution begins.