#include <algorithm>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <deque>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define FOR(i,k,n) for(int(i)=(k);(i)<(n);++(i))
#define rep(i,n) FOR(i,0,n)
#define all(v) begin(v),end(v)
#define debug(x) std::cerr<<#x<<": "<<x<<"\n"
#define debug2(x,y) std::cerr<<#x<<": "<<x<<", "<<#y<<": "<<y<<"\n"
#define debug3(x,y,z) std::cerr<<#x<<": "<<x<<", "<<#y<<": "<<y<<", "<<#z<<": "<<z<<"\n"

using ll=long long;
using vi=std::vector<int>;
using vvi=std::vector<vi>;
using vll=std::vector<ll>;
using vvll=std::vector<vll>;
template<typename T> using vvec=std::vector<std::vector<T>>;
template<typename T>
auto make_v(size_t sz){return std::vector<T>(sz);}
template<typename T,typename... Ts>
auto make_v(size_t sz,Ts...ts){return std::vector<decltype(make_v<T>(ts...))>(sz,make_v<T>(ts...));}
template<typename T>
void fill_v(T&var,const T&x){var=x;}
template<typename V,typename T>
void fill_v(V&v,const T&x){for(auto&& w:v){fill_v(w,x);}}

template<typename T> std::ostream& operator<<(std::ostream&s,const std::vector<T>&v){
  int sz=v.size();s<<"\n";rep(i,sz){s<<v[i];if(i<sz-1){s<<"\t";}}s<<"\n";return s;}
template<typename T> std::ostream& operator<<(std::ostream&s,const std::vector<std::vector<T>>&v){
  for(auto&& w:v){s<<w;}return s;}
template<typename T> std::ostream& operator<<(std::ostream&s,const std::deque<T>&v){
  int sz=v.size();s<<"\n";rep(i,sz){s<<v[i];if(i<sz-1){s<<"\t";}}s<<"\n";return s;}
template<typename T> std::ostream& operator<<(std::ostream&s,const std::deque<std::deque<T>>&v){
  for(auto&& w:v){s<<w;}return s;}
template<typename T> std::ostream& operator<<(std::ostream&s, const std::set<T>&v){
  s<<"\n";for(auto&& elm:v){s<<elm<<"\t";}s<<"\n";return s;}

inline void scan(int&a){scanf("%d",&a);}
inline void scan(ll&a){scanf("%lld",&a);}
inline void scan(char&a){scanf(" %c",&a);}
inline void scan(double&a){scanf("%lf",&a);}
inline void scan(std::string&s){char BUF[3000000];scanf(" %s",BUF);s=std::string(BUF);}
template<typename T>
inline void scan(std::vector<T>&v){for(auto&& sv:v){scan(sv);}}
template<typename First,typename...Args>
inline void scan(First&f,Args&...args){scan(f);scan(args...);}
inline void print(int a){printf("%d\n",a);}
inline void print(ll a){printf("%lld\n",a);}
inline void print(double a){printf("%.12f\n",a);}
inline void print(std::string s){std::cout<<s<<"\n";}

using namespace std;

vvi g;

vi search(deque<vi> & que, set<int> & visited) {
  vi path;
  while (!que.empty()) {
    auto seq = que.front();
    que.pop_front();
    int u = seq.back();
    int sz = seq.size();
    for (auto v : g[u]) {
      rep (i, sz) {
        if (v == seq[i]) {
          path.insert(begin(path), begin(seq) + i, end(seq));
          break;
        }
      }
      if (!path.empty()) {
        break;
      }
      seq.push_back(v);
      que.push_back(seq);
      seq.pop_back();
      visited.insert(v);
    }
    if (!path.empty()) {
      break;
    }
  }
  return path;
}

int main() {
  int n, m;
  scan(n, m);
  g.resize(n);
  vi deg_in(n, 0);
  vi a(m);
  vi b(m);
  rep (i, m) {
    scan(a[i], b[i]);
    --a[i]; --b[i];
    g[a[i]].push_back(b[i]);
    deg_in[b[i]] += 1;
  }
  if (n == 1) {
    print(-1);
    return 0;
  }

  set<int> visited;
  deque<vi> que;
  rep (i, n) {
    if (deg_in[i] == 0) {
      que.push_back((vi){i});
      visited.insert(i);
    }
  }
  if (que.empty()) {
    que.push_back((vi){0});
    visited.insert(0);
  }

  vi path;
  while (true) {
    path = search(que, visited);
    if (!path.empty()) {
      break;
    }
    que.clear();
    if ((int)visited.size() < n) {
      rep (i, n) {
        if (visited.find(i) == end(visited)) {
          que.push_back((vi){i});
          visited.insert(i);
          break;
        }
      }
    } else {
      print(-1);
      return 0;
    }
  }


  while (true) {
    int sz = path.size();
    set<int> paths;
    for (int u : path) {
      paths.insert(u);
    }
    vi num2index(n, -1);
    rep (i, sz) {
      num2index[path[i]] = i;
    }
    vi new_path;
    rep (i, m) {
      if (paths.find(a[i]) == end(paths) || paths.find(b[i]) == end(paths)) {
        continue;
      }
      int index_a = num2index[a[i]];
      int index_b = num2index[b[i]];
      if ((index_a + 1) % sz == index_b) {
        continue;
      }
      if (index_a < index_b) {
        new_path.insert(end(new_path), begin(path), begin(path) + index_a + 1);
        new_path.insert(end(new_path), begin(path) + index_b, end(path));
      } else {
        new_path.insert(end(new_path), begin(path) + index_b, begin(path) + index_a + 1);
      }
      break;
    }
    if (new_path.empty()) {
      break;
    } else {
      swap(path, new_path);
    }
  }


  int sz = path.size();
  print(sz);
  rep (i, sz) {
    print(path[i] + 1);
  }

  return 0;
}


Measured memory_bytes: 512