/*------------------------------------------------------------------*/
/*------------------------- begin template -------------------------*/
/*------------------------------------------------------------------*/

// ref: tanakh <https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8>
// diff: using Parser
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut parser = Parser::from_str($s);
        input_inner!{parser, $($r)*}
    };
    (parser = $parser:ident, $($r:tt)*) => {
        input_inner!{$parser, $($r)*}
    };
    (new_stdin_parser = $parser:ident, $($r:tt)*) => {
        let stdin = std::io::stdin();
        let reader = std::io::BufReader::new(stdin.lock());
        let mut $parser = Parser::new(reader);
        input_inner!{$parser, $($r)*}
    };
    ($($r:tt)*) => {
        input!{new_stdin_parser = parser, $($r)*}
    };
}

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

macro_rules! read_value {
    ($parser:ident, ( $($t:tt),* )) => {
        ( $(read_value!($parser, $t)),* )
    };
    ($parser:ident, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($parser, $t)).collect::<Vec<_>>()
    };
    ($parser:ident, chars) => {
        read_value!($parser, String).chars().collect::<Vec<char>>()
    };
    ($parser:ident, char_) => {
        read_value!($parser, String).chars().collect::<Vec<char>>()[0]
    };
    ($parser:ident, usize1) => {
        read_value!($parser, usize) - 1
    };
    ($parser:ident, i64_) => {
        $parser.fast_i64()
    };
    ($parser:ident, usize_) => {
        $parser.fast_i64() as usize
    };
    ($parser:ident, usize1_) => {
        ($parser.fast_i64() - 1) as usize
    };
    ($parser:ident, $t:ty) => {
        $parser.next::<$t>().expect("Parse error")
    };
}

use std::io;
use std::io::BufRead;
use std::str;

// ref: tatsuya6502 <https://qiita.com/tatsuya6502/items/cd448486f7ef7b5b8c7e>
// ref: wariuni <https://qiita.com/tatsuya6502/items/cd448486f7ef7b5b8c7e#comment-7040a5ae96305e884eb9>
// diff: using std::io::BufRead::fill_buf()
pub struct Parser<R> {
    pub reader: R,
    buf: Vec<u8>,
    pos: usize,
}

impl Parser<io::Empty> {
    pub fn from_str(s: &str) -> Parser<io::Empty> {
        Parser {
            reader: io::empty(),
            buf: s.as_bytes().to_vec(),
            pos: 0,
        }
    }
}

impl<R: BufRead> Parser<R> {
    pub fn new(reader: R) -> Parser<R> {
        Parser {
            reader: reader,
            buf: vec![],
            pos: 0,
        }
    }
    pub fn update_buf(&mut self) {
        self.buf.clear();
        self.pos = 0;
        loop {
            let (len, complete) = {
                let buf2 = self.reader.fill_buf().unwrap();
                self.buf.extend_from_slice(buf2);
                let len = buf2.len();
                (len, buf2[len - 1] <= 0x20)
            };
            self.reader.consume(len);
            if complete {
                break;
            }
        }
    }
    pub fn next<T: str::FromStr>(&mut self) -> Result<T, T::Err> {
        loop {
            let mut begin = self.pos;
            while begin < self.buf.len() && (self.buf[begin] <= 0x20) {
                begin += 1;
            }
            let mut end = begin;
            while end < self.buf.len() && (self.buf[end] > 0x20) {
                end += 1;
            }
            if begin != self.buf.len() {
                self.pos = end;
                return unsafe { str::from_utf8_unchecked(&self.buf[begin..end]) }.parse::<T>();
            } else {
                self.update_buf();
            }
        }
    }
    pub fn fast_i64(&mut self) -> i64 {
        loop {
            let mut begin = self.pos;
            while begin < self.buf.len() && (self.buf[begin] <= 0x20) {
                begin += 1;
            }
            if begin == self.buf.len() {
                self.update_buf();
                continue;
            }
            let mut res = 0;
            let (is_positive, mut end) = match self.buf[begin] {
                b'+' => (true, begin + 1),
                b'-' => (false, begin + 1),
                _ => (true, begin),
            };
            while end < self.buf.len() && (self.buf[end] > 0x20) {
                res = res * 10 + (self.buf[end] as i64 - '0' as i64);
                end += 1;
            }
            if begin != self.buf.len() {
                self.pos = end;
                return if is_positive { res } else { -res };
            } else {
                self.update_buf();
            }
        }
    }
}

#[allow(unused_imports)]
use std::cmp::*;
use std::collections::BTreeMap;

fn main() {
    input! {
        new_stdin_parser = parser,
    }
    let res = solve(&mut parser);
    print!("{}", res);
}

/*------------------------------------------------------------------*/
/*-------------------------- end template --------------------------*/
/*------------------------------------------------------------------*/

use std::fmt::Write;

fn dfs1(
    v: usize,
    edges: &Vec<Vec<usize>>,
    ids: &mut Vec<Option<usize>>,
    next_id: &mut usize,
    hist: &mut Vec<usize>,
) -> bool {
    if ids[v] == Some(edges.len()) {
        return true;
    }
    if ids[v].is_some() {
        return false;
    }
    ids[v] = Some(edges.len());
    hist.push(v);
    for &next in &edges[v] {
        if dfs1(next, edges, ids, next_id, hist) {
            return true;
        }
    }
    hist.pop();
    ids[v] = Some(*next_id);
    *next_id += 1;
    return false;
}

fn g(n: usize, es: &Vec<(usize, usize)>) -> Vec<usize> {
    let mut edges = vec![vec![]; n];
    for &(s, t) in es.iter() {
        edges[s].push(t);
    }
    let mut ids = vec![None; n];
    let mut next_id = 0;
    for v in 0..n {
        let mut hist = vec![];
        if dfs1(v, &edges, &mut ids, &mut next_id, &mut hist) {
            return hist;
        }
    }
    vec![]
}

fn solve<R: BufRead>(parser: &mut Parser<R>) -> String {
    let mut buf = String::new();
    input! {
        parser = parser,
        n: usize,
        m: usize,
        es: [(usize1,usize1); m],
    }
    let mut res = g(n, &es);
    if res.is_empty() {
        writeln!(buf, "-1");
        return buf;
    }
    'l1: loop {
        // eprintln!("res = {:?}", res);
        let mut nexts = vec![0; n];
        for i in 0..res.len() {
            nexts[res[i]] = res[(i + 1) % res.len()];
        }
        let mut map = BTreeMap::new();
        for (i, &r) in res.iter().enumerate() {
            map.insert(r, i);
        }
        let mut res2 = vec![];
        for &(s, t) in &es {
            if map.contains_key(&s) && map.contains_key(&t) && nexts[s] != t {
                let si = map[&s];
                let ti = map[&t];
                // eprintln!("si = {}, ti = {}", si, ti);
                if si < ti {
                    for i in 0..si + 1 {
                        res2.push(res[i]);
                    }
                    for i in ti..res.len() {
                        res2.push(res[i])
                    }
                    res = res2;
                    continue 'l1;
                } else {
                    for i in 0..res.len() {
                        if ti <= i && i <= si {
                            res2.push(res[i]);
                        }
                    }
                    res = res2;
                    continue 'l1;
                }
            }
        }
        break;
    }
    writeln!(buf, "{}", res.len());
    for &r in &res {
        writeln!(buf, "{}", r + 1);
    }
    /*
    let mut edges = vec![vec![]; n];
    for &(s,t) in &es {
        edges[s].push(t);
    }
    let mut ids = vec![None; n];
    let mut next_id = 0;
    for v in 0..n {
        let mut hist = vec![];
        if dfs1(v, &edges, &mut ids, &mut next_id, &mut hist) {
            writeln!(buf, "{}", hist.len());
            for &h in &hist {
                writeln!(buf, "{}", h + 1);
            }
            return buf;
        }
    }
    */
    buf
}


Measured memory_bytes: 4352