题解 | #汽水瓶#

import sys

def get_spe(n):
    if( n == 1):
        return 0
    if(n == 2):
        return 1
    num = int(n/3)
    return  num + get_spe(num + n%3)

while True:
    try:
        s = int(input())
        if s == 0:
            break
        print( int(get_spe(s)))
    except:
        break