fn main() {
    let mut nk = String::new();
    std::io::stdin().read_line(&mut nk).ok();
    let nk: Vec<usize> = nk.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let n = nk[0];
    let k = nk[1];

    if k < n/2+n%2 {
        println!("-1");
        return;
    }
    let mut result = vec![];
    for i in 0..k {
        let lidx = if i >= n/2 && n % 2 == 0 { 1 } else { 0 } + 2*i;
        let lidx = lidx % n;
        let ridx = (lidx + 1) % n;
        result.push((lidx+1, ridx+1));
    }
    for i in 0..n {
        result.push((i+1, i+1));
    }
    println!("{}", result.len());
    for &(l, r) in result.iter() {
        println!("{} {}", l, r);
    }
}