Bulls and Cows is an old code-breaking paper and pencil game for two players, predating the
similar commercially-marketed board game Master Mind. It is a game with numbers that may
date back a century or more, played by two opponents. One player becomes the code maker,
the other the code breaker. The code maker has a 5-digit secret number (the digits must be all
different). Then, the code breaker tries to guess the secret number.
For every guess, the code maker gives a hint: if the matching digits are on their right positions,
they are "bulls", if on different positions, they are "cows".