import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

def my_callback(channel):
	print('You pressed the button')

GPIO.setup(21, GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.add_event_detect(21, GPIO.FALLING, callback=my_callback)

i = 0

while True:
	i = i + 1
	print(i)
	time.sleep(1)