Player can press left arrow and right arrow button to change direction
Player can press SPACE to generate a shell.
Once the enemy ship is attacked by the shell, a "bomb" can be heared. Once one single enemy ship is attacked by three times, then it will go back to the origin (0, 0)
The score is to calculate how many times you attack the enemies. Once it reaches 100, then you win the game
Once an enemy reaches the bottom and you fail to reach 100, then you lose
Once an enemy reaches the bottom and you fail to reach 100, then you lose