#include <iostream>

using namespace std;

int main(void) {
  long long int N;
  cin >> N;
  cout << (N - 1) / 2 << endl;
}

### Prompt

Your task is to create a cpp solution to the following problem:

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.

Examples

Input

4


Output

1


Input

999999


Output

499999

### Response

```cpp
#include <iostream>

using namespace std;

int main(void) {
  long long int N;
  cin >> N;
  cout << (N - 1) / 2 << endl;
}

```