题目描述
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
输入格式
见题面
输出格式
见题面
样例输入
4
样例输出
1