Bulls and Cows

Rules of the "Bulls and Cows" game are very simple:
You can ask your phone if your four digit code is correct and you can receive the answer from your phone how many bulls and cows do you have now.
The number of the Bulls means how many digits in your code number are correct and stay in correct positions in the phone code number.
The number of the Cows means how many digits in your code number are correct but stay in wrong positions in the phone code number.
And analizing this data you have to find out a guessing code number of your phone. Good Luck!