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 Rust (1.42.0) (judge verdict: Wrong Answer)

#![allow(unused_imports)]

use text_io::*;
use proconio::*;
use std::collections::*;
use itertools::Itertools;
use std::process::exit;
use std::cmp::*;
use num::*;
use num::integer::Roots;

fn main() {
    input! {
    a:usize,
    }
    println!("{}",(a+1)/2+1)
}