p02865 NIKKEI Programming Contest 2019-2 - Sum of Two Integers
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
```python
N = int(input())
print(N // 2 - int(N // 2 * 2 == N))
```