import sys

for line in sys.stdin:
    a = int(line.rstrip())
    if a == 0:
        break
    n=0
    while a>=2:
        if a==2:
            a=a+1
        n += a//3
        a = a//3 + a % 3
    print(n)

感觉 while里的计算有点跛脚，但是耽误太多时间在这就先这样吧