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)

use proconio::input;
// use std::collections::HashMap;

#[allow(non_snake_case)]
fn main() {
    input! {
        N: i32,
    }

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

}