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: Accepted)

n = gets.chomp.to_i
map = Hash.new(false)
count = 0
(1...n).each do |i|
  break if map[i]
  break if map[n - i]
  break if i == n - i
  map[i] = true
  map[n - i] = true
  count += 1
end
puts count