题解 | #汽水瓶#

from math import floor
def func(n):
    if n>1:
        l = list(range(1,floor(n/2+1)))
        l = l*2
        l.append(0)
        l=sorted(l)[:-1]
        print(l[-1])
    elif n==1:
        print(0)
while True:
    try:
        n = int(input())
        func(n)
    except:
        break
        

from math import floor
while True:
    try:
        n = int(input())
        if n>0:
            print(floor(n/2))
    except:
        break