Actually, you have to use the computer with the C twice, as the hidden sequence tells you.
First: to even start the input sequence, when no other computer is accessible (even though the C is not yet displayed, it is the same computer)
Second: as the last input of the sequence
Once you relize, the sequence needs the C two times and you can only enter "y" in every computer once, after the very first input, this is actually a very logical solution.