Submission #69909411

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Source Code Expand

#![allow(unused_imports,non_snake_case,dead_code)]
use std::{cmp::Reverse as Rev,ops::Range,collections::*,iter::*,mem::swap};
use rustc_hash::{FxHashSet as HashSet,FxHashMap as HashMap};
use ac_library::{*,modint::ModIntBase,ModInt998244353 as M};
use {proconio::{marker::*,*},itertools::*};



#[fastout]
fn main(){
    input!{
        n:usize,
    }

    if n<=5{
        println!("-1");
        return;
    }

    let mut es=vec![];

    let n0=n/2*2;
    let mut pair=vec![];
    for i in 0..n0/2{
        pair.push((i,n0-1-i));
    }

    for i in 0..pair.len(){
        for j in i+1..pair.len(){
            es.push((pair[i].0,pair[j].1));
            es.push((pair[i].1,pair[j].0));
        }
    }

    if n%2==1{
        let last=n-1;
        for i in 0..pair.len(){
            es.push((pair[i].0,last));
        }
    }

    println!("{}",es.len());
    for &(u,v) in &es{
        println!("{} {}",u+1,v+1);
    }
}

Submission Info

Judge Result