#[allow(unused_imports)]
use std::char::*;
#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
use std::io::*;
use std::str::FromStr;

#[allow(unused_macros)]
macro_rules! debug {($($a:expr),*) => {println!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);}}
#[allow(unused_macros)]
macro_rules! input { ( source = $ s : expr , $ ( $ r : tt ) * ) => { let mut iter = $ s . split_whitespace ( ) ; let mut next = || { iter . next ( ) . unwrap ( ) } ; input_inner ! { next , $ ( $ r ) * } } ; ( $ ( $ r : tt ) * ) => { let stdin = std :: io :: stdin ( ) ; let mut bytes = std :: io :: Read :: bytes ( std :: io :: BufReader :: new ( stdin . lock ( ) ) ) ; let mut next = move || -> String { bytes . by_ref ( ) . map ( | r | r . unwrap ( ) as char ) . skip_while ( | c | c . is_whitespace ( ) ) . take_while ( | c |! c . is_whitespace ( ) ) . collect ( ) } ; input_inner ! { next , $ ( $ r ) * } } ; }
#[allow(unused_macros)]
macro_rules! input_inner { ( $ next : expr ) => { } ; ( $ next : expr , ) => { } ; ( $ next : expr , $ var : ident : $ t : tt $ ( $ r : tt ) * ) => { let $ var = read_value ! ( $ next , $ t ) ; input_inner ! { $ next $ ( $ r ) * } } ; }
#[allow(unused_macros)]
macro_rules! read_value { ( $ next : expr , ( $ ( $ t : tt ) ,* ) ) => { ( $ ( read_value ! ( $ next , $ t ) ) ,* ) } ; ( $ next : expr , [ $ t : tt ; $ len : expr ] ) => { ( 0 ..$ len ) . map ( | _ | read_value ! ( $ next , $ t ) ) . collect ::< Vec < _ >> ( ) } ; ( $ next : expr , chars ) => { read_value ! ( $ next , String ) . chars ( ) . collect ::< Vec < char >> ( ) } ; ( $ next : expr , usize1 ) => { read_value ! ( $ next , usize ) - 1 } ; ( $ next : expr , $ t : ty ) => { $ next ( ) . parse ::<$ t > ( ) . expect ( "Parse error" ) } ; }

#[derive(PartialEq, PartialOrd)]
pub struct Total<T>(pub T);
impl<T: PartialEq> Eq for Total<T> {}
impl<T: PartialOrd> Ord for Total<T> { fn cmp(&self, other: &Total<T>) -> std::cmp::Ordering { self.0.partial_cmp(&other.0).unwrap() }}


#[allow(dead_code)]
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

#[allow(dead_code)]
fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split_whitespace()
        .map(|e| e.parse().ok().unwrap()).collect()
}


#[allow(unused_imports)]
use std::f64;

#[allow(dead_code)]
fn min<T: PartialOrd>(a:T, b:T) -> T {
    if a < b { a }
    else { b }
}
#[allow(dead_code)]
fn max<T: PartialOrd>(a:T, b:T) -> T {
    if a < b { b }
    else { a }
}

use std::hash::Hash;
#[allow(dead_code)]
fn inc<T: Hash+Ord>(map:&mut BTreeMap<T,i64>, key:T) {
    let count = map.entry(key).or_insert(0);
    *count += 1;
}

#[allow(dead_code)]
fn digit(ch: char) -> i64 {
    return (ch as u8 - b'0') as i64
}

#[allow(dead_code)]
const MAX:usize = 1000006;
#[allow(dead_code)]
const INF:i64 = std::i64::MAX;
#[allow(dead_code)]
const MOD:i64 = 1e9 as i64 + 7;

struct UnionFind {
    p: Vec<usize>,
    rank: Vec<usize>
}

impl UnionFind {
    fn new(n:usize) -> Self {
        UnionFind {
            p: (0..n).collect(),
            rank: vec![0;n]
        }
    }

    fn find_set(&mut self, x:usize) -> usize {
        if x != self.p[x] {
            let q = self.p[x];
            self.p[x] = self.find_set(q);
        }
        self.p[x]
    }

    fn same(&mut self, x:usize, y:usize) -> bool {
        self.p[x] == self.p[y]
    }

    fn link(&mut self, x:usize, y:usize) {
        if self.rank[x] > self.rank[y] {
            self.p[y] = x;
        } else {
            self.p[x] = y;
            if self.rank[x] == self.rank[y] {
                self.rank[y] += 1;
            }
        }
    }

    fn unite(&mut self, x:usize, y:usize) {
        let (root_x,root_y) = (self.find_set(x), self.find_set(y));
        self.link(root_x,root_y);
    }

}

fn main() {
    input!{
        n: usize,
    }

    let mut uf = UnionFind::new(n);

    if n%2==0 {
        for i in 0..n/2 {
            //debug!(i,n-i-1);
            uf.unite(i, n-i-1);
        }
    } else {
        for i in 0..n/2 {
            //debug!(i,n-i-2);
            uf.unite(i, n-i-2);
        }
    }

    let mut v = vec![];
    for i in 0..n {
        for j in i+1..n {
            if uf.same(i, j) { continue; }
            v.push((i,j));
        }
    }

    println!("{}", v.len());
    for (s,t) in v {
        println!("{} {}", s+1,t+1);
    }

}
/*



*/


Measured memory_bytes: 4352