题解 | #汽水瓶#

import sys
def compute(num1):
    global drinknum
    if num1 <2:
        return 0
    if num1 == 2:
        drinknum +=1
    else:
        tamp1 = num1 //3 
        drinknum += tamp1
        compute(tamp1+num1%3)
for line in sys.stdin:
    a = int(line.strip())
    if a == 0: break
    drinknum = 0
    compute(a)
    print(drinknum)