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

int32_t main()
{
  ll n;
  cin>>n;
  cout<<(n-1)/2;
  return 0;
}
### Prompt

Please create a solution in Cpp 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<bits/stdc++.h>
using namespace std;
#define ll long long

int32_t main()
{
  ll n;
  cin>>n;
  cout<<(n-1)/2;
  return 0;
}
```