Contest Duration: - (local time) (120 minutes) Back to Home
A - Sum of Two Integers /

Time Limit: 2 sec / Memory Limit: 1024 MB

• 1 ≦ N ≦ 10^6
• N は整数である。

入力

N


入力例 1

4


出力例 1

1


入力例 2

999999


出力例 2

499999


Score : 100 points

Problem Statement

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


Sample Input 1

4


Sample Output 1

1


There is only one way to choose two distinct integers totaling 4: to choose 1 and 3. (Choosing 3 and 1 is not considered different from this.)

Sample Input 2

999999


Sample Output 2

499999