Problem:
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

Output

Print the answer.

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

Solution in Python (3.4.3) (judge verdict: Accepted)

n=int(input())

if n==0 or n==1 or n==2:
  print(0)
elif n==3:
  print(1)
elif n%2==1:#奇数
  print(int(n/2))
elif n%2!=1:#偶数
  print(int(n/2)-1)