#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <cstdio>
#include <cmath>
#include <tuple>
#include <iomanip>
#include <numeric>
#include <unordered_map>
#include <sstream>
#include<limits.h>
#include<float.h>
#include<list>
#include <array>
#include <complex>
#include<stdio.h>
#include<string.h>
#include <bitset>
// #include<assert.h>
#include<random>
using namespace std;
#define int long long

#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807LL
#define I64_MAX2 1223372036854775807LL
#define INF I64_MAX2
#define MOD 1000000007
// #define MOD 998244353 
#define MEM_SIZE 100010
#define DEBUG_OUT true
#define ALL(x) (x).begin(), (x).end()

template<typename T> void DEBUG(T e){if(DEBUG_OUT == false)return; std::cout << e <<" ";}
template<typename T> void DEBUG(const std::vector<T>& v){if(DEBUG_OUT == false)return;for(const auto& e : v){std::cout<< e << " "; } std::cout << std::endl;}
template<typename T> void DEBUG(const std::vector<std::vector<T> >& vv){if(DEBUG_OUT == false)return;for(const auto& v : vv){ DEBUG(v); } }
template<class T,class... Ts> void DEBUG(T d, Ts... e){if(DEBUG_OUT == false)return;DEBUG(d);DEBUG(e...);}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; abort();}}
template< typename T1, typename T2 > inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template< typename T1, typename T2 > inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

//https://qiita.com/drken/items/a803d4fc4a727e02f7ba#4-6-%E3%82%B5%E3%82%A4%E3%82%AF%E3%83%AB%E6%A4%9C%E5%87%BA

vector<int> bfs (int sv,vector<vector<int> > & GRAPH)
{
  vector<int> dist(GRAPH.size(),INF);
  vector<int> pre(GRAPH.size(),-1);
  queue<int> q;
  dist[sv] = 0;
  q.push(sv);
  while(!q.empty())
  {
    int v = q.front();q.pop();
    for (auto &&u : GRAPH[v])
    { 
        if(dist[u] != INF)continue;
        dist[u] = dist[v] + 1;
        pre[u] = v;
        q.push(u);
    }
  }

  pair<int,int> best(INF,-1);
  for (int v = 0; v < GRAPH.size(); v++)
  {
    if(v == sv)continue;
    for (auto &&u : GRAPH[v])
    {
      if(u == sv)
      {
        best = min(best,make_pair(dist[v],v));
      }
    }
    
  }
  if(best.first == INF) return vector<int> (GRAPH.size()+1);
  vector<int> res;
  int v = best.second;
  while(v != -1){
    res.push_back(v);
    v = pre[v];
  }
  return res;
  
}

//-----CODE------//
void solve(void)
{
  int n,m;
  cin>>n>>m;
  vector<vector<int> > GRAPH (n);
  for (int i = 0; i < m; i++)
  {
    int A,B;
    cin>>A>>B;
    A--;B--;
    GRAPH[A].push_back(B);
  }
  vector<int> ans(n + 1);
  for (int i = 0; i < n; i++)
  {
    vector<int> now = bfs(i,GRAPH);
    if(now.size() < ans.size()) ans = now;
  }
  if(ans.size() == n+1)
  {
    cout<<-1<<endl;
    return;
  }
  else
  {
    cout<<ans.size()<<endl;
    for (auto &&v : ans)
    {
      cout<<v + 1<<endl;
    }
    
  }
  

  return;
}

int32_t main(int32_t argc, const char *argv[])
{
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(11);
  solve();

  return 0;
}


Measured memory_bytes: 3760