import threading

x = 0
lock = threading.Lock()

def incr():
    global x, lock
    for i in range(100):
        with lock:
            xx = x
        xx += 1
        with lock:
            x = xx

threads = [threading.Thread(target=incr) for _ in range(20)]
for t in threads:
    t.start()
for t in threads:
    t.join()

print(x)