#include <bits/stdc++.h>
using namespace std;
#define ll          long long
#define ld          long double

void dfs(ll x, ll p, vector<ll>&par, vector<ll>&vis, vector<vector<ll>>&g, vector<vector<bool>>&ed)
{
	vis[x]=1;
	par[x]=p;
	for(ll y : g[x])
	{
		if(vis[y]==1)
		{
			set<ll>cyc;
			ll cur=x;
			while(cur!=y)
			{
				cyc.insert(cur);
				cur=par[cur];
			}
			cyc.insert(cur);
			bool ok=1;
			// only el of set
			ll n=g.size()-1;
			for(ll z : cyc)
			{
				ll cnt=0;
				for(ll i=1; i<=n; i++) if(cyc.count(i)) cnt+=ed[z][i];
				if(cnt>1) {ok=0; break;}
			}
			if(ok)
			{
				cout<<cyc.size()<<endl;
				for(ll z : cyc) cout<<z<<endl;
				exit(0);
			}
		}
		else 
		{
			dfs(y, x, par, vis, g, ed);
		}
	}
	vis[x]=2;
}

void solve()  
{
	ll n, m; cin>>n>>m;
	vector<vector<ll>>g(n + 1);
	vector<vector<bool>>ed(n+1, vector<bool>(n+1, 0));
	for(ll i=0; i<m; i++) { ll x, y; cin>>x>>y; g[x].push_back(y); ed[x][y]=1; }
	vector<ll>par(n+1);
	vector<ll>vis(n+1, 0);
	for(ll i=1; i<=n; i++) if(vis[i]==0) { vis[i]=1; dfs(i, i, par, vis, g, ed); }
	cout<<-1<<endl;
}

int main() {
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);  
	ll T = 1;
	//cin>>T; 
	while(T--)
	    solve();
	return 0;
}
