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.to_i

array = (1..n-1).to_a
array_r = (1..n-1).to_a.reverse

#p array
#p array_r

ctn = 0
for i in 0..array.length-1
  #puts array[i]
  #puts array_r[i]
  if array[i] != array_r[i]
    ctn += 1
   # puts "a"
  end
end

puts ctn / 2