#include <iostream>
using namespace std;

int main()
{
    int N;
    cin >> N;
    cout << (N-1)/2 << endl;
    return 0;
}
### Prompt

Construct a CPP code solution to the problem outlined:

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()
{
    int N;
    cin >> N;
    cout << (N-1)/2 << endl;
    return 0;
}
```