#include <bits/stdc++.h>

int main() {
    int n; scanf("%d", &n);
    printf("%d\n", (n-1)/2);
    //int amo; std::cin >> amo;
}

### Prompt

Create a solution in Cpp for 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>

int main() {
    int n; scanf("%d", &n);
    printf("%d\n", (n-1)/2);
    //int amo; std::cin >> amo;
}

```