没能发现整除的规律，所以写了个递归，被输入输出的\n小坑了一下导致第一次做题时没通过。。

import sys

def cal(n,maxm):
    if n>=3:
        yu = n%3
        ex = int((n-yu)/3)
        if yu+ex==2:
            yu+=1
        n = yu+ex
        maxm += ex
        maxm = cal(n, maxm)
    return maxm

if __name__=='__main__':
    for line in sys.stdin:
        if line.replace("\n","")=='0':break
        n = int(line)
        maxm = cal(n, 0)
        print(maxm)