Bulls and Cows is played against the computer. The computer thinks of a number, while you tries to guess it.

• The number to be guessed must be a 4 digit number, using only digits from 1 - 9, each digit at most once. e.g. 1234 is valid, 0123 is not valid, 9877 is not valid, 9876 is valid.
• For every guess that the player makes, he gets 2 values - the number of bulls and the number of cows. 1 bull means the guess contains and the target number have 1 digit in common, and in the correct position. 1 cow means the guess and the target have 1 digit in common, but not in correct position. e.g. Let the target be 1234. Guessing 4321 will give 0 bulls and 4 cows. 3241 will give 1 bull and 3 cows. 4 bulls means you have won the game!

P.S. If you still have doubts about the rules, check it over here: Visit wikipedia