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 Julia (0.5.0) (judge verdict: Accepted)

#=
A:
- Julia version: 
- Author: abap
- Date: 2019-11-09
=#

parseInt(x) = parse(Int,x)

x = parseInt(readline())
if x % 2 == 0
    println(div(x,2)-1)
else
    println(div(x,2))
end