use std::str::FromStr;

const DX: [i32; 8] = [0, 1, 0, -1, 1, 1, -1, -1];
const DY: [i32; 8] = [1, 0, -1, 0, 1, -1, -1, 1];

fn main() {
    let (r, w) = (std::io::stdin(), std::io::stdout());
    let mut sc = IO::new(r.lock(), w.lock());

    let mut n: i64 = sc.read();
    let mut mt = n / 3;
    while (n - mt * 3) % 2 != 0 {
        mt -= 1;
    }
    println!("{}", (n - mt * 3) / 2);
}

pub struct IO<R, W: std::io::Write>(R, std::io::BufWriter<W>);

impl<R: std::io::Read, W: std::io::Write> IO<R, W> {
    pub fn new(r: R, w: W) -> Self { Self(r, std::io::BufWriter::new(w)) }
    pub fn write<S: ToString>(&mut self, s: S) {
        use std::io::Write;
        self.1.write_all(s.to_string().as_bytes()).unwrap();
    }
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        use std::io::Read;
        let buf = self.0.by_ref().bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b' ' || b == b'\n' || b == b'\r' || b == b'\t')
            .take_while(|&b| b != b' ' && b != b'\n' && b != b'\r' && b != b'\t')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }.parse().ok().expect("Parse error.")
    }
    pub fn chars(&mut self) -> Vec<char> { self.read::<String>().chars().collect() }
    pub fn usize0(&mut self) -> usize { self.read::<usize>() - 1 }
    pub fn vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }
}