C. Find Amir

C. Find Amir

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

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  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 .

 

超级简单啊啊啊啊啊啊

//
//  main.cpp
//  C. Find Amir
//
//  Created by 徐智豪 on 2017/5/4.
//  Copyright © 2017年 徐智豪. All rights reserved.
//

#include <iostream>
using namespace std;
int n;
int vis[100005]={0};
int main(int argc, const char * argv[]) {
    cin>>n;
    cout<<n/2+(n%2==0?-1:0)<<endl;
    return 0;
}