An old Paper and Pencil game - You have to guess a 4 digits (all different) number, chosen by computer .
If the matching digits are in their right positions, they are "bulls", if in different positions, they are "cows".
Example:
Secret number: 4271
You chose: 1234
Answer: 1 bull and 2 cows. (The bull is "2", the cows are "4" and "1".)