/*input
4 5
1 2
2 3
2 4
4 1
4 3

*/
 
/*
                                                    ______________
                                                          |       )
                                                          |   )   /
                                                          |______/
                                                    |     |      \
                                                    |     |   )   \
                                                    |_____|_______)
 
*/
 
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <ctime>
#include <cassert>
#include <complex>
#include <string>
#include <cstring>
#include <chrono>
#include <random>
#include <queue>
#include <bitset>
#include <iomanip>
 
using namespace std;
 
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> PII;
typedef pair<ll , ll> PLL;
typedef long double ld;
 
#define pb push_back
#define all(c) c.begin(),c.end()
#define allr(c) c.rbegin(),c.rend()
int mod = 1000000007;
const int inf = 1034567891;
const ll LL_INF = 1234567890123456789ll;
#define PI 3.14159265
#define endl '\n'
#define F first
#define S second
#define debug(x) cout << #x << " = " << x << endl;
#define TRACE
 
#ifdef TRACE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
  cout << name << " : " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
  const char* comma = strchr(names + 1, ',');cout.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}
#else
#define trace(...)
#endif
 
#define out(container) for (auto it : container) cout << it << " "; cout << endl;
 
 
template < typename T > T GCD(T a, T b)            { ll t; while(a) { t = a; a = b % a; b = t; } return b; }
template < typename T > string toString(T a)       { return to_string(a); }
template < typename T > void toInt(string s, T &x) { stringstream str(s); str >> x;}
inline int add(int x, int y){ x += y; if(x >= mod) x -= mod; return x;}
inline int sub(int x, int y){ x -= y; if(x < 0) x += mod; return x;}
inline int mul(int x, int y){ return (x * 1ll * y) % mod;}
inline int powr(int a, ll b){
  int x = 1 % mod;
  while(b){
    if(b & 1) x = mul(x, a);
    a = mul(a, a);
    b >>= 1;
  }
  return x;
}
inline int inv(int a){ return powr(a, mod - 2);}

const int N = 1e3 + 5;

int n, m;
vector <int> adj[N];

int main()
{
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  
  cin >> n >> m;
  int u, v;
  for (int i = 0; i < m; i++) {
    cin >> u >> v;
    adj[u].pb(v);
  }
  vector <int> ans;
  for (int i = 1; i <= n; i++) {
    queue <int> qu;
    vector <int> par(n + 1, -1);
    vector <bool> vis(n + 1, false);
    for (auto it : adj[i]) {
      par[it] = i;
      qu.push(it);
    }
    while (!qu.empty()) {
      int s = qu.front();
      qu.pop();
      vis[s] = true;
      for (auto it : adj[s]) {
        if (it == i) {
          vector <int> temp;
          while (s != i) {
            temp.pb(s);
            s = par[s];
          }
          temp.pb(i);
          if (ans.size() == 0 || ans.size() > temp.size()) ans = temp;
          break;
        } else if (par[it] == -1) {
          par[it] = s;
          qu.push(it);
        }
      }
    }
  }

  if (ans.size() == 0) {
    cout << -1 << endl;
  } else {
    cout << ans.size() << endl;
    for (auto it : ans) {
      cout << it << endl;
    }
  }

  return 0;
}

Measured memory_bytes: 384