import pygame



def run_game():
    
    ## Initialize the pygame submodules and set up the display window.
    pygame.init()

    width = 640
    height = 480
    game_window = pygame.display.set_mode((width,height))

    # load the balloon picture and convert it into Pygame's internal
    # image format
    balloon = pygame.image.load("balloon.gif")
    balloon = balloon.convert()
    balloon_w = balloon.get_width()
    balloon_h = balloon.get_height()
    balloon_x = 100
    balloon_y = 100
    balloon_vx = 0.5
    balloon_vy = 0.5
    
    ## The game loop starts here.
    keepGoing = True    
    while (keepGoing):

        ## Handle events.
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                keepGoing = False

        balloon_x += balloon_vx
        balloon_y += balloon_vy
        if balloon_x > width - balloon_w:
            balloon_x = width - balloon_w
            balloon_vx = -balloon_vx
        elif balloon_x < 0:
            balloon_x = 0
            balloon_vx = -balloon_vx
        if balloon_y > height - balloon_h:
            balloon_y = height - balloon_h
            balloon_vy = -balloon_vy
        elif balloon_y < 0:
            balloon_y = 0
            balloon_vy = -balloon_vy


        ## Draw picture and update display
        game_window.fill(pygame.color.Color("darkblue"))

        game_window.blit(balloon, (balloon_x,balloon_y))

        pygame.display.update()


    ## The game loop ends here.
    pygame.quit()


## Call the function run_game.
run_game()