#[allow(unused_imports)]
use std::cmp::{Ordering};

#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};

#[allow(unused_imports)]
use std::io::{stdin, stdout, BufWriter, Write};

#[allow(unused_imports)]
use std::iter::FromIterator;

#[allow(unused_macros)]
macro_rules! min {
    ($x: expr) => ($x);
    ($x: expr, $($z: expr),+) => {{
        std::cmp::min($x, min!($($z),*))
    }}
}

#[allow(unused_macros)]
macro_rules! max {
    ($x: expr) => ($x);
    ($x: expr, $($z: expr),+) => {{
        std::cmp::max($x, max!($($z),*))
    }}
}

#[allow(unused_macros)]
macro_rules! get { 
    ([$t: ty]) => { 
        { 
            let mut line = String::new(); 
            stdin().read_line(&mut line).unwrap(); 
            line.split_whitespace().map(|t|t.parse::<$t>().unwrap()).collect::<Vec<_>>()
        }
    };
    ([$t: ty];$n: expr) => {
        (0..$n).map(|_|get!([$t])).collect::<Vec<_>>()
    };
    ($t: ty) => {
        {
            let mut line = String::new();
            stdin().read_line(&mut line).unwrap();
            line.trim().parse::<$t>().unwrap()
        }
    };
    ($($t: ty),*) => {
        { 
            let mut line = String::new();
            stdin().read_line(&mut line).unwrap();
            let mut iter = line.split_whitespace();
            ($(iter.next().unwrap().parse::<$t>().unwrap(),)*)
        }
    };
    ($t: ty; $n: expr) => {
        (0..$n).map(|_|get!($t)).collect::<Vec<_>>()
    };
    ($($t: ty),*; $n: expr) => {
        (0..$n).map(|_|get!($($t),*)).collect::<Vec<_>>()
    };
}

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        #[cfg(debug_assertions)]
        writeln!(&mut std::io::stderr(), concat!("[DEBUG] ", $(stringify!($a), "={:?} "),*), $($a),*).unwrap();
    }
}

const BIG_STACK_SIZE: bool = true;

#[allow(dead_code)]
fn main() {
    use std::thread;
    if BIG_STACK_SIZE {
        thread::Builder::new()
            .stack_size(32 * 1024 * 1024)
            .name("solve".into())
            .spawn(solve)
            .unwrap()
            .join()
            .unwrap();
    } else {
        solve();
    }
}

fn solve() {
    let (n,k) = get!(usize, usize);

    if k > (n-1)*(n-2)/2 {
        println!("-1");
        return;
    }

    let mut set = vec![HashSet::new(); n+1];
    let mut count = 0;
    for i in 3..n+1 {
        count += 1;
        if count > k {
            break;
        }
        set[2].insert(i);
        set[i].insert(2);
    }
    for i in 3..n+1 {
        for j in (i+1)..n+1 {
            count += 1;
            if count > k {
                break;
            }
            set[i].insert(j);
            set[j].insert(i);
        }
    }
    println!("{}", n*(n-1)/2-set.iter().map(|ref e| e.iter().count()).sum::<usize>()/2);

    let mut queue = VecDeque::new();
    let mut visited = vec![vec![false; n+1];n+1];
    queue.push_back(1);

    while let Some(node) = queue.pop_front() {
        for i in 1..n+1 {
            if i == node {
                continue;
            }
            if set[node].contains(&i) {
                continue;
            }
            if visited[node][i] {
                continue;
            }
            visited[node][i] = true;
            visited[i][node] = true;
            queue.push_back(i);
            println!("{} {}", node, i);
        }
    }
}


Measured memory_bytes: 8572