#include <cstdio>
using namespace std;
int n;
int main(){
    scanf("%d",&n);
    printf("%d\n",(n-1)>>1);
}

### 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 <cstdio>
using namespace std;
int n;
int main(){
    scanf("%d",&n);
    printf("%d\n",(n-1)>>1);
}

```