#[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,m) = get!(usize, usize);
    let ab = get!(usize, usize; m);

    let mut nodes = vec![Vec::new(); n+1];
    let mut inb = vec![0usize; n+1];
    let mut list = (1..n+1).collect::<BTreeSet<_>>();
    for (a,b) in ab {
        nodes[a].push(b);
        inb[b] += 1;
        list.remove(&b);
    }

    let mut list = list.into_iter().collect::<VecDeque<_>>();
    let mut unselected = (1..n+1).collect::<BTreeSet<_>>();

    while let Some(node) = list.pop_front() {
        unselected.remove(&node);
        for &next in &nodes[node] {
            inb[next] -= 1;
            if inb[next] == 0 {
                list.push_back(next);
            }
        }
    }

    if unselected.len() == 0 {
        println!("-1");
        return;
    }

    let mut mn = (1..n+1).collect::<Vec<_>>();
    for i in 1..n+1 {
        let mut queue = VecDeque::new();
        let mut visited = vec![None; n+1];
        queue.push_back(i);
        visited[i] = Some(0);
        while let Some(node) = queue.pop_front() {
            let mut flag = false;
            for &next in &nodes[node] {
                if next == i {
                    visited[i] = Some(node);
                    flag = true;
                    break;
                }
                if visited[next].is_some() {
                    continue;
                }
                visited[next] = Some(node);
                queue.push_back(next);
            }
            if flag {
                break;
            }
        }
        if visited[i].unwrap() == 0 {
            continue;
        }
        let mut current = i;
        let mut res = Vec::new();
        while let Some(next) = visited[current] {
            res.push(next);
            if next == i {
                break;
            }
            current = next;
        }
        if mn.len() > res.len() {
            mn = res;
        }
    }
    println!("{}", mn.len());
    for e in mn {
        println!("{}",e);
    }
}


Measured memory_bytes: 8572