#include <iostream>

using namespace std;

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

Please formulate 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() {
	int n; cin >> n;
	cout << n / 2 - (n & 1 ? 0 : 1) << endl;
	
	return 0;
}
```