#include<bits/stdc++.h>
using namespace std;

int main(){
	int N;scanf("%d",&N);
	printf("%d\n",(N-1)/2);

	return 0;
}
### Prompt

In cpp, your task is to solve 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<bits/stdc++.h>
using namespace std;

int main(){
	int N;scanf("%d",&N);
	printf("%d\n",(N-1)/2);

	return 0;
}
```