#include <iostream>
#include <vector>
#include <queue>

using namespace std;

vector<int> g[1005];

int main() {
  int n,m;
  cin >> n >> m;
  for(int i = 0; i < m; i++) {
    int a,b;
    cin >> a >> b;
    --a;--b;
    g[a].push_back(b);
  }

  vector<int> ans(n+1);
  int M = ans.size();
  
  for(int s = 0; s < n; s++) {
    vector<int> dist(n,-1);
    vector<int> prev(n,-1);
    queue<int> q;
    dist[s] = 0;
    q.push(s);
    while(!q.empty()) {
      int v = q.front();
      q.pop();
      for(int i = 0; i < g[v].size(); i++) {
	int u = g[v][i];
	if(dist[u] != -1) continue;
	dist[u] = dist[v] + 1;
	prev[u] = v;
	q.push(u);
      }
    }

    for(int t = 0; t < n; t++) {
      if(t == s || dist[t] == -1) continue;
      for(int i = 0; i < g[t].size(); i++) {
	int u = g[t][i];
	if(u == s) {
	  vector<int> tmp;
	  tmp.push_back(s);
	  int v = t;
	  while(v != s) {
	    tmp.push_back(v);
	    v = prev[v];
	  }
	  if(M > tmp.size()) {
	    M = tmp.size();
	    ans = tmp;
	  }
	}
      }
    }
  }

  if(M == n + 1) {
    cout << -1 << endl;
    return 0;
  }
  else {
    cout << M << endl;
    for(int i = 0; i < ans.size(); i++) {
      cout << ans[i] + 1 << endl;
    }
  }
}
