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::str::FromStr;
use std::fmt::Debug;
use std::cmp;

#[allow(dead_code)]
fn read_as_vec<T>() -> Vec<T>
where T: FromStr, <T as FromStr>::Err : Debug{
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    s.trim().split_whitespace().map(|c| T::from_str(c).unwrap()).collect()

}

#[allow(dead_code)]
fn read_as_string() -> String{
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    s.trim().to_string()
}

struct UnionFind{
    v: Vec<usize>
}

impl UnionFind{
    fn new(n: usize) -> UnionFind{
        let mut v: Vec<usize> = Vec::new();
        for i in 0..n{
         v.push(i);
        }
        UnionFind{v:v}
    }

    fn find(&mut self, i: usize) -> usize{
        if self.v[i] == i{
            i
        }else{
            let ni = self.v[i];
            self.v[i] = self.find(ni);
            self.v[i]
        }
    }

    fn union(&mut self, i: usize, j: usize){
        let root_i = self.find(i);
        let root_j = self.find(j);
        self.v[root_i] = root_j;
    }

}

struct CombMod{
    fac: Vec<u64>,
    inv: Vec<u64>,
    finv: Vec<u64>,
    mmod: u64,
}

impl CombMod{
    pub fn new(max: u64, m: u64) -> CombMod{
        let max: usize = max as usize;
        let mut fac: Vec<u64> = vec![0;max+1];
        let mut finv: Vec<u64> = vec![0;max+1];
        let mut inv: Vec<u64> = vec![0;max+1];

        fac[0] = 1;
        fac[1] = 1;
        finv[0] = 1;
        finv[1] = 1;
        inv[1] = 1;

        for i in 2..max+1{
            fac[i] = fac[i-1] * i as u64 % m;
            inv[i] = m - inv[m as usize %i] * (m/(i as u64)) % m;
            finv[i] = finv[i-1] * inv[i] % m;
        }

        CombMod{
            fac: fac,
            finv: finv,
            inv: inv,
            mmod: m,
        }
    }

    fn get(&self, n: u64, m: u64) -> u64{
        assert!(n >= m && n != 0 && m != 0);
        self.fac[n as usize] * self.finv[m as usize] % self.mmod
            * self.finv[(n-m) as usize] % self.mmod
    }
}

fn main(){
    let n: u64 = read_as_vec()[0];

    println!("{}", (n-1)/2);
}