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);

    if n == 2 {
        writeln!(writer, "1").ok();
        writeln!(writer, "1 2").ok();
        return;
    }

    let mut sum = 1;
    let mut i = 2;
    while sum + i <= n {
        sum += i - 1;
        i += 1;
    }
    sum += 1;
    i -= 1;
    
    let mut ans = Vec::new();
    let mut p = 1;
    let mut j = 2;
    for k in (2..=i).rev() {
        for _ in 0..k - if k == i { 0 } else { 1 } {
            ans.push((p, j));
            j += 1;
        }
        p = j - 1;
    }

    for _ in 0..n - sum {
        ans.push((p, j));
        p = j;
        j += 1;
    }

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