import threading

counter = 0
def runner(N):
    global counter
    for i in range(N):
        counter += 1

def main():
    N = 1000000
    t1 = threading.Thread(target=runner, args=(N,))
    t2 = threading.Thread(target=runner, args=(N,))
    t1.start()
    t2.start()

    t1.join()
    t2.join()
    print(f"expected: {N+N}, calculated: {counter}")

if __name__ == '__main__':
    main()