A few years ago Sajjad left his school and register to another one due to security reasons. Now he wishes to find Amir, one of his schoolmates and good friends.

There are n schools numerated from 1 to n. One can travel between each pair of them, to do so, he needs to buy a ticket. The ticker between schools i and j costs <image> and can be used multiple times. Help Sajjad to find the minimum cost he needs to pay for tickets to visit all schools. He can start and finish in any school.

Input

The first line contains a single integer n (1 ≤ n ≤ 105) — the number of schools.

Output

Print single integer: the minimum cost of tickets needed to visit all schools.

Examples

Input

2


Output

0


Input

10


Output

4

Note

In the first example we can buy a ticket between the schools that costs <image>.
# Author Name: Ajay Meena
# Codeforce : https://codeforces.com/profile/majay1638
# Codechef  : https://www.codechef.com/users/majay1638
# import inbuilt standard input output
import sys
import math
from sys import stdin, stdout


def get_ints_in_variables():
    return map(int, sys.stdin.readline().strip().split())


def get_ints_in_list(): return list(
    map(int, sys.stdin.readline().strip().split()))


def get_string(): return sys.stdin.readline().strip()


def Solution(s):
    pass


def main():
    # //TAKE INPUT HERE
    # op = []
    n=int(input())-1
    res=n//2
    print(res)
    
    # print(op)


#  call the main method  pa
if __name__ == "__main__":
    main()
