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: Accepted)

#![allow(non_snake_case, unused)]
use proconio::{input};
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
use std::cmp::{max, min};
use std::cmp::Ordering::{self, Greater, Less};

fn main() {
    input![
	n: i32,
    ];

    let mut ans = n / 2;
    if n % 2 == 0 {
	ans -= 1;
    }
    println!("{}", ans);
}