#include <bits/stdc++.h>
#define LLI long long int
#define FOR(v, a, b) for(LLI v = (a); v < (b); ++v)
#define FORE(v, a, b) for(LLI v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(LLI v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define fst first
#define snd second
#define popcount __builtin_popcount
#define UNIQ(v) (v).erase(unique(ALL(v)), (v).end())
#define bit(i) (1LL<<(i))

#ifdef DEBUG
#include <misc/C++/Debug.cpp>
#else
#define dump(...) ((void)0)
#endif

#define gcd __gcd

using namespace std;
template <class T> constexpr T lcm(T m, T n){return m/gcd(m,n)*n;}

template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}

template <typename T, typename U> bool chmin(T &a, const U &b){return (a>b ? a=b, true : false);}
template <typename T, typename U> bool chmax(T &a, const U &b){return (a<b ? a=b, true : false);}
template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}

struct Init{
  Init(){
    cin.tie(0);
    ios::sync_with_stdio(false);
  }
}init;

template <typename Cost = int> class Edge{
public:
  int from,to;
  Cost cost;
  Edge() {}
  Edge(int to, Cost cost): to(to), cost(cost){}
  Edge(int from, int to, Cost cost): from(from), to(to), cost(cost){}

  Edge rev() const {return Edge(to,from,cost);}
  
  friend ostream& operator<<(ostream &os, const Edge &e){
    os << "(FROM: " << e.from << "," << "TO: " << e.to << "," << "COST: " << e.cost << ")";
    return os;
  }
};

template <typename T> using Graph = vector<vector<Edge<T>>>;
template <typename T> using Tree = vector<vector<Edge<T>>>;

template <typename C, typename T> void add_edge(C &g, int from, int to, T w){
  g[from].push_back(Edge<T>(from, to, w));  
}

template <typename C, typename T> void add_undirected(C &g, int a, int b, T w){
  g[a].push_back(Edge<T>(a, b, w));
  g[b].push_back(Edge<T>(b, a, w));
}

template <typename T>
struct DirectShortestCycle{
  int N;
  bool no_cycle;
  vector<int> cycle;

  void bfs(int i, const Graph<T> &g, int &min_len){
    queue<int> q;
    q.push(i);

    vector<int> dist(N), pre(N);
    vector<bool> visited(N);

    while(not q.empty()){
      int cur = q.front(); q.pop();

      if(visited[cur]) continue;
      visited[cur] = true;

      for(auto &e : g[cur]){
        if(e.to == i){
          no_cycle = false;
          if(dist[cur] < min_len){
            min_len = dist[cur];

            cycle.clear();

            int j = cur;
            while(1){
              cycle.push_back(j);
              if(j == i) break;
                
              j = pre[j];
            }

            reverse(ALL(cycle));
          }

          return;
        }
        if(not visited[e.to]){
          dist[e.to] = dist[cur] + 1;
          pre[e.to] = cur;
          q.push(e.to);
        }
      }
    }
  }
  
  DirectShortestCycle(const Graph<T> &g): N(g.size()), no_cycle(true){
    int min_len = INT_MAX;

    REP(i,N){
      bfs(i,g,min_len);
    }
  }
};

int main(){
  int N,M;
  while(cin >> N >> M){

    Graph<int> g(N);
    REP(i,M){
      int a,b; cin >> a >> b;
      --a, --b;
      add_edge(g, a, b, 1);
    }

    auto res = DirectShortestCycle<int>(g);
    
    
    if(res.no_cycle){
      cout << -1 << endl;
    }else{
      cout << res.cycle.size() << endl;
      for(auto x : res.cycle){
        cout << x+1 << endl;
      }
    }
  }

  return 0;
}


Measured memory_bytes: 384