Number Waterfall - Add Games

## Description

Simulate a waterfall using numbers. Waterfall is considered well-formed only if:
\- Sum of the numbers is equal to the goal.
\- Marked numbers should be adjacent horizontal, vertical & diagonally. That is, no empty line union.
\- 2 numbers marked on each row.

Also available a challenges section, where different variations appear. In both cases, there are two levels: normal & difficult. In the normal level, the sum is shown, however, in the difficult level, you will have to add & subtract mentally because it disappears.

Challenges:
\- 15s & 30s countdown
\- 6 hidden cells
\- Random, Negative, Pair & Odd numbers
\- Multiples of 10

Add & Subtract. Brain challenges game.

For Apple Watch, reduced version.