Problem:
Score : 100 points

Problem Statement

How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

1 \leq N \leq 10^6

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

4

Sample Output 1

1

There is only one way to choose two distinct integers totaling 4: to choose 1 and 3. (Choosing 3 and 1 is not considered different from this.)

Sample Input 2

999999

Sample Output 2

499999

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

//
//  main.cpp
//  Study
//
//  Created by 佐々木勇星 on 2020/03/12.
//  Copyright © 2020 佐々木勇星. All rights reserved.
//

#include <iostream>
#include <vector>
#include <cmath>
#include <sstream>
#include <iomanip>
#include <algorithm>
#include <map>

using namespace std;


int main(int argc, const char * argv[]) {

    int n;
    cin >> n;
    
    cout << ((n + 2 - 1) / 2) - 1 << endl;
    
    return 0;
}