The rules are simple:
1. Connect tiles so that the their sums match one of the goal values (e.g. 4 = 2 + 2, 7 = 9 - 2).
2. Round tiles can be used more then once.
3. Some levels also require that all cells in the grid must be filled.
Controls

Use the mouse to navigate and select. Click and hold to drag connections.

Hotkeys:
Space, U - Undo last move
R, Enter, Backspace - Reset current level
Esc - Return to the main menu
M, S - Toggle music and sound effects