#include <iostream>
using namespace std;
int main()
{
	long long a;
	cin>>a;
	cout<<(a-1)/2<<endl;
 } 
### Prompt

Please provide a CPP coded solution to the problem described below:

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()
{
	long long a;
	cin>>a;
	cout<<(a-1)/2<<endl;
 } 
```