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)

use std::io;
use io_ext::Reader;

fn main() {
    let stdin = io::stdin();
    let mut r = Reader::new(io::BufReader::new(stdin.lock()));
    let n: u64 = r.read_line().parse().unwrap();
    println!("{}", (n - 1) / 2);
}

/// Extension of io module.
pub mod io_ext {
    use std::io::BufRead;

    pub struct Reader<R> {
        buf: String,
        inner: R,
    }

    impl<R> Reader<R> {
        #[inline]
        pub fn new(inner: R) -> Self {
            Reader { buf: String::new(), inner: inner }
        }

        #[inline]
        pub fn into_inner(self) -> R {
            self.inner
        }
    }

    impl<R: BufRead> Reader<R> {
        #[allow(deprecated)]
        #[inline]
        pub fn read_line(&mut self) -> &str {
            self.buf.clear();
            self.inner.read_line(&mut self.buf).unwrap_or_else(|e| panic!("{}", e));
            self.buf.trim_right()
        }
    }
}

/// Module supporting to parse strings.
pub mod parse {
    use std::str::FromStr;
    use std::borrow::Borrow;

    pub trait ParseNext: Iterator {
        fn parse_next<F: FromStr>(&mut self) -> F;
    }

    impl<'a, S: Borrow<str>, I: Iterator<Item=S>> ParseNext for I {
        fn parse_next<F: FromStr>(&mut self) -> F {
            if let Some(s) = self.next() {
                match s.borrow().parse() {
                    Ok(x) => x,
                    Err(_) => panic!("provided string cannot be parsed")
                }
            } else {
                panic!("iterator has no next element")
            }
        }
    }
}