题解 | #汽水瓶#

import sys

def purchase(n):
    return n//3,n%3+n//3

while True:
    n = int(input())
    if n == 0:
        break
    else:
        sum = 0
        while n>=2:
            if n>2:
                a,n = purchase(n)
            else:
                a,n = 1,0
            sum += a
        if n <=1:
            print(sum)
            continue