Easy math game for kids.
Given random four digit integer, e.g. 6591.
Subtract one from each digit, output four new digits, i.e. 5480.
Write it as fast as you can in 60 seconds.
Get as many as correct answers.
Correct answer, earn plus 1 point.
Incorrect answer, deduct minus 1 point.
Brain, eyes and finger productivity.
Leaderboard is given by the Game Center.

Include Add by one and leaderboard.
Given random four digit integer, e.g. 6591.
Add one from each digit, output four new digits, i.e. 7602.