fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: usize = n.trim().parse().unwrap();

    let mut result = vec![];
    for i in (0..n).rev() {
        for j in n-i-1..i {
            result.push((j+1, i+1));
        }
    }
    println!("{}",result.len());
    for &(u, v) in result.iter() {
        println!("{} {}", u, v);
    }
}