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 Ruby (2.3.3) (judge verdict: Time Limit Exceeded)

n = gets.to_i
result = 0
(1..n/2).each{|i|
  (1...n).each{|j|
    next if i == j
    result += 1 if i + j == n
  }
}
puts result