import threading


lock = threading.Lock()


def add():
    global num
    lock.acquire()
    num += 1
    lock.release()


def sub():
    global num
    lock.acquire()
    num -= 1
    lock.release()

for i in range(1000):
    num = 0
    t1 = threading.Thread(target=add, )
    t2 = threading.Thread(target=sub, )
    t1.start()
    t2.start()
    t1.join()
    t2.join()
print(num)