Heart10 is simple apple watch game using Tens Complement.

The "Tens Complement" is the number we add to make 10.
Apple Watch ask numbers for you, then you input very first
number’s complement to make 10.
if it is correct answer then asking number disappear.
if you can’t remove numbers in time, game will be over.

Tens Complements makes your mental arithmetic a whole lot easier.
This will be very good brain training. Have fun!