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 (1.15.1) (judge verdict: Accepted)

macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {($(read_value!($iter, $t)),*)};
    ($iter:expr, [ $t:tt ; $len:expr ]) => {(0..$len).map(|_|read_value!($iter, $t)).collect::<Vec<_>>()};
    ($iter:expr, chars) => {read_value!($iter, String).chars().collect::<Vec<char>>()};
    ($iter:expr, usize1) => {read_value!($iter, usize) - 1};
    ($iter:expr, $t:ty) => {$iter.next().unwrap().parse::<$t>().expect("Parse error")};
}

fn main() {
    input! {n: i32}
    println!("{}", n / 2 + (n % 2) - 1);
}