题解 | #汽水瓶#

while True:
    try:
        a = int(input())
        b = 0
        c = 0
        d = a
        if d > 0:
            while d > 2:
                b = b + int(d/3)
                d = int(d/3) + d%3
            if d == 2:
                print(b+1)
            else:
                print(b)
    except:
        break