My trying to make the program decrement every 1 second. It keeps printing 99. What did I do wrong?

# SimpleGUI program template

# Import the module
import simplegui

# Define global variables (program state)
countdown = 100
# Define "helper" functions
def blast_off():
    global countdown
    countdown = 100 - 1
    print countdown

# Define event handler functions


# Create a frame

# Register event handlers
timer = simplegui.create_timer(1000,blast_off)
# Start frame and timers
timer.start()