#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($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)*}
    };
}

#[allow(unused_macros)]
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)*}
    };

    ($next:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

#[allow(unused_macros)]
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>>()
    };

    ($next:expr, bytes) => {
        read_value!($next, String).into_bytes()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

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

#[allow(unused_imports)]
use std::cmp::{max, min};
#[allow(unused_imports)]
use std::collections::{BinaryHeap, HashMap};

#[allow(dead_code)]
type Graph = Vec<Vec<usize>>;

fn main() {
    input! {
        n: usize,
        m: usize,
        edges: [(usize1, usize1); m],
    }

    let mut graph = vec![vec![]; n];

    for &(a, b) in &edges {
        graph[a].push(b);
    }

    let mut ans = vec![];

    'outer: for i in 0..n {
        let mut cache = vec![None; n];
        let mut queue = std::collections::VecDeque::new();
        queue.push_front((i, 0, Some(n)));
        // println!("{}", i);
        while !queue.is_empty() {
            let (cur, cost, pre) = queue.pop_back().unwrap();
            if cache[cur].is_some() {
                continue;
            }
            cache[cur] = pre;
            // println!("{} {}", cur, cost);

            for &next in &graph[cur] {
                if cache[next].is_some() {
                    if next == i {
                        let mut vv = vec![];
                        let mut v = cur;
                        for _ in 0..cost + 1 {
                            vv.push(v + 1);
                            v = cache[v].unwrap();
                        }
                        if ans.len() == 0 || vv.len() < ans.len() {
                            ans = vv;
                        }
                        continue 'outer;
                    } else {
                        continue;
                    }
                }
                queue.push_front((next, cost + 1, Some(cur)));
            }
        }
    }

    if ans.len() == 0 {
        println!("-1");
    } else {
        println!("{}", ans.len());
        for v in &ans {
            println!("{}", v);
        }
    }
}


Measured memory_bytes: 4352