Bulls and Cows is an old, numerical version of code-breaking game for two players.

It is played with 4 digits. The digits must be all different. Then, in turn, 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"(green circle here), if in different positions, they are "cows"(orange circle here).

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

You must follow rules listed below:

• Zero can not be used as a first numeral of the whole number
• Same numeral can not be used twice
• Quantity of numerals must be 4, not more, not less
• You type your guess, then opponent's matching by turns