math game

On a sheet of paper, the players each write a 4-digit secret number. The digits must be all different. Then, in turn, the players try to guess their opponent's number who gives the number of matches. If the matching digits are in their right positions, they are "bulls", if in different positions, they are "cows". Example:



Secret number: 4271



Opponent's try: 1234



Answer: 1 bull and 2 cows. (The bull is "2", the cows are "4" and "1".)



The first player to reveal the other's secret number wins the game