#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
use std::io::{Write, BufWriter};
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    ($($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)*}
    };
}

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)*}
    };
}

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, [ $t:tt ]) => {{
        let len = read_value!($next, usize);
        (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    }};

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused)]
macro_rules! debug {
    ($($format:tt)*) => (write!(std::io::stderr(), $($format)*).unwrap());
}
#[allow(unused)]
macro_rules! debugln {
    ($($format:tt)*) => (writeln!(std::io::stderr(), $($format)*).unwrap());
}

// Reference: https://atcoder.jp/contests/abc142/submissions/7748110
fn solve() {
    let out = std::io::stdout();
    let mut out = BufWriter::new(out.lock());
    macro_rules! puts {
        ($($format:tt)*) => (write!(out,$($format)*).unwrap());
    }
    input! {
        n: usize, m: usize,
        ab: [(usize1, usize1); m],
    }
    const INF: usize = 1 << 40;
    let mut mi = (INF, vec![]);
    let mut g = vec![vec![]; n];
    for &(a, b) in &ab {
        g[a].push(b);
    }
    for &(a, b) in &ab {
        // Shortest path b -> a
        let mut que = VecDeque::new();
        let mut dist = vec![INF; n];
        let mut inv = vec![INF; n];
        que.push_back((b, 0, INF));
        while let Some((v, d, prev)) = que.pop_front() {
            if dist[v] <= d { continue; }
            dist[v] = d;
            inv[v] = prev;
            for &w in &g[v] {
                que.push_back((w, d + 1, v));
            }
        }
        let mut path = vec![];
        if dist[a] < INF {
            let mut cur = a;
            while cur != b {
                path.push(cur);
                cur = inv[cur];
            }
            path.push(b);
            path.reverse();
            mi = min(mi, (dist[a], path));
        }
    }
    if mi.0 >= INF {
        puts!("-1\n");
    } else {
        puts!("{}\n", mi.1.len());
        for v in mi.1 {
            puts!("{}\n", v + 1);
        }
    }
}

fn main() {
    // In order to avoid potential stack overflow, spawn a new thread.
    let stack_size = 104_857_600; // 100 MB
    let thd = std::thread::Builder::new().stack_size(stack_size);
    thd.spawn(|| solve()).unwrap().join().unwrap();
}


Measured memory_bytes: 8572