use std::io::Write;

fn main() {
    let ref mut buf = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), buf).ok();
    let mut iter = buf.split_whitespace();

    macro_rules! scan {
        ([$t:tt; $n:expr]) => ((0..$n).map(|_| scan!($t)).collect::<Vec<_>>());
        (($($t:tt),*)) => (($(scan!($t)),*));
        (Usize1) => (scan!(usize) - 1);
        (Bytes) => (scan!(String).into_bytes());
        ($t:ty) => (iter.next().unwrap().parse::<$t>().unwrap());
    }

    let stdout = std::io::stdout();
    let mut writer = std::io::BufWriter::new(stdout.lock());

    let n = scan!(usize);

    let mut ans = Vec::new();
    for u in 1..n {
        for v in u + 1..=n + 1 - u {
            ans.push((u, v));
        }
    }

    writeln!(writer, "{}", ans.len()).ok();
    for (u, v) in ans {
        writeln!(writer, "{} {}", u, v).ok();
    }
}