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)

def nikkei2019_2_a
    n = gets.chomp.to_i

    cnt = 0
    for i in 1..n/2
        j = n - i
        cnt += 1 if i > 0 and j > 0 and i != j
    end

    puts cnt
end

nikkei2019_2_a