You need to fight the invaders coming from the right of the screen. You use the aim (on left side) to shift the numbers 1-9 and "n" in turn to fight the number on the right. The "." button of the calculator is to "Aim" while the "+" button is to "Shoot".
Once the total of the numbers you fired is a multiple of 10 , an award "n" will appear. You have to shoot the "n" by turning the aim to "n" to get this award. You will have a higher score for an award. Only 10 numbers will be displayed on screen at the same time. You have to shoot all the approaching numbers before they coming to the left , otherwise you used a "try" . You have 3 tries for each game . The bar at the right side of the aim showing how many try you have left. Once you level up , the speed of approaching invaders will be slightly increased.
Enjoy it!

source : itunes.apple.com