#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for(int i=(int)(s-1); i>=(int)(e); i--)
#define pb push_back
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()
#define fi first
#define se second

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll INF = 1e9;
const ll MOD = 1e9 + 7;
const double EPS = 1e-8;

template<typename T> T chmax(T& a, const T& b){return a = (a > b ? a : b);}
template<typename T> T chmin(T& a, const T& b){return a = (a < b ? a : b);}

#define DEBUG_MODE
#ifdef DEBUG_MODE
#define dump(x) cout << #x << " : " << x << " "
#define dumpL(x) cout << #x << " : " << x << '\n'
#define LINE cout << "line : " << __LINE__ << " "
#define LINEL cout << "line : " << __LINE__ << '\n'
#define dumpV(v) cout << #v << " : ["; for(auto& t : v) cout << t << ", "; cout<<"]" << " "
#define dumpVL(v) cout << #v << " : ["; for(auto& t : v) cout << t << ", "; cout<<"]" << endl
#define STOP assert(false)
#else
#define dump(x) 
#define dumpL(x) 
#define LINE 
#define LINEL 
#define dumpV(v) 
#define dumpVL(v) 
#define STOP assert(false)
#endif
#define mp make_pair
 
namespace std {
template<class S, class T>
ostream &operator <<(ostream& out, const pair<S, T>& a) {
    out << '(' << a.fi << ", " << a.se << ')';
    return out;
}
}

//有向、無向グラフ共通クラス(隣接リスト)
struct Graph {
    int n;
    using WEIGHT_TYPE = long long;
    static const WEIGHT_TYPE INF = 1e18;
    struct Edge {
        int to;
        WEIGHT_TYPE weight;
    };
    struct Edge2 {
        int from;
        int to;
        WEIGHT_TYPE weight;
    };
    vector<vector<Edge>> es;
    Graph(int n) : n(n), es(n) {}

    // dijkstra O(E log V)
    vector<WEIGHT_TYPE> dijkstra(int s) {
        vector<WEIGHT_TYPE> d(n, INF);
        d[s] = 0;
        using P = pair<WEIGHT_TYPE, int>;
        priority_queue<P, vector<P>, greater<P> > q;
        q.push({0LL, s});
        while (!q.empty()) {
            auto p = q.top(); q.pop();
            int cur = p.second;
            auto cost = p.first;
            if (d[cur] < p.first) continue;
            for (auto& e : es[cur]) {
                int to = e.to;
                auto dist = e.weight + cost;
                if (dist < d[to]) {
                    d[to] = dist;
                    q.push({dist, to});
                }
            }
        }
        return d;
    }
    // warshall_floyd O(n^3)
    vector<vector<WEIGHT_TYPE>> warshall_floyd() {
        vector<vector<WEIGHT_TYPE>> d(n, vector<WEIGHT_TYPE>(n, INF));
        rep(i, n) d[i][i] = 0LL;
        rep(i, n) for (auto && e : es[i]) {
            int j = e.to;
            chmin(d[i][j], e.weight);
        }
        rep(k, n) rep(i, n) rep(j, n) {
            chmin(d[i][j], d[i][k] + d[k][j]);
        }
        return d;
    }
    // 頂点sから到達できるか
    vector<bool> getVisitable(int s) {
        vector<bool> ret(n);
        queue<int> q;
        q.push(s);
        ret[s] = true;
        while(!q.empty()) {
            auto cur = q.front();
            q.pop();
            for(auto&& e : es[cur]) {
                if(!ret[e.to]) {
                    ret[e.to] = true;
                    q.push(e.to);
                }
            }
        }
        return ret;
    }
    //2部グラフ判定
    bool isBipartile() {
        vector<int> memo(n, -1);
        rep(i, n) {
            if(memo[i] != -1) continue;
            queue<int> q;
            q.push(i);
            memo[i] = 0;
            while(!q.empty()) {
                auto v = q.front();
                q.pop();
                for(auto&& e: es[v]) {
                    auto u = e.to;
                    if(memo[u] == -1) {
                        memo[u] = !memo[v];
                        q.push(u);
                    }
                    else if(memo[u] == memo[v]) {
                        return false;
                    }
                }
            }
        }
        return true;
    }
};

// 無向グラフ
struct GraphUD : public Graph {
    GraphUD(int n) : Graph(n) {}
    void add_edge(int from, int to, WEIGHT_TYPE weight) {
        es[from].push_back({to, weight});
        es[to].push_back({from, weight});
    }
    vector<Edge2> getEdge2() {
        vector<Edge2> ret;
        rep(i, n) for (auto && e : es[i]) {
            if (i < e.to) ret.push_back({i, e.to, e.weight});
        }
        return ret;
    }
    // 橋の検出
    // http://nupioca.hatenadiary.jp/entry/2013/11/03/200006
    // Calculate bridges in a undirected graph.
    // Assume graph is connected and has no parallel edges or self-loops.
    vector<Edge2> getBridges()
    {
        int V = n;
        // res: bridges
        vector<Edge2> res;
        // assume at least the first vertex exists
        vector<int> low(V, -1); // lowest reacheable index
        vector<int> pre(V, -1); // pre-order index
        int count = 0;          // pre-order index counter

        // v: current node
        // from: parent node
        function<int(int, int)> dfs = [&](int v, int from) {
            pre[v] = count++;
            low[v] = pre[v];
            for (auto &&e : es[v])
            {
                int to = e.to;
                if (pre[to] == -1)
                {
                    // destination has not been visited
                    // visit destination and update low[v]
                    low[v] = min(low[v], dfs(to, v));
                    if (low[to] == pre[to])
                    {
                        // edge is not contained in a closed path -> bridge
                        res.push_back({v, to, e.weight});
                    }
                }
                else
                {
                    if (from == to)
                    {
                        // ignore a path to parent
                        continue;
                    }
                    low[v] = min(low[v], low[to]);
                }
            }
            return low[v];
        };

        dfs(0, -1); // start dfs from vertex 0

        return res;
    }
};


// 有向グラフ
struct GraphD : public Graph {
    GraphD(int n) : Graph(n) {}
    void add_edge(int from, int to, WEIGHT_TYPE weight) {
        es[from].push_back({to, weight});
    }
    vector<Edge2> getEdge2() {
        vector<Edge2> ret;
        rep(i, n) for (auto && e : es[i]) {
            ret.push_back({i, e.to, e.weight});
        }
        return ret;
    }
    GraphD getReverseGraph() {
        GraphD g(n);
        rep(i, n) for (auto && e : es[i]) {
            g.add_edge(e.to, i, e.weight);
        }
        return g;
    }
    vector<vector<int>> scc() {
        vector<vector<int>> res;
        vector<int> cmp(n);
        vector<int> vs;
        vector<vector<int>> r_es(n);
        rep(i, n) for (auto && e : es[i]) {
            int j = e.to;
            r_es[j].push_back(i);
        }

        vector<bool> used(n);
        function<void(int)> dfs = [&](int v) {
            used[v] = true;
            for (auto && e : es[v]) {
                int to = e.to;
                if (!used[to]) dfs(to);
            }
            vs.push_back(v);
        };
        function<void(int, int)> rdfs = [&](int v, int k) {
            used[v] = true;
            cmp[v] = k;
            for (auto && to : r_es[v]) {
                if (!used[to]) rdfs(to, k);
            }
        };

        fill(all(used), 0);
        vs.clear();
        for (int v = 0; v < n; v++) {
            if (!used[v]) dfs(v);
        }
        fill(all(used), 0);
        int k = 0;
        for (int i = vs.size() - 1; i >= 0; i--) {
            if (!used[vs[i]]) rdfs(vs[i], k++);
        }
        res.clear();
        res.resize(k);
        for (int i = 0; i < n; i++) {
            res[cmp[i]].push_back(i);
        }
        return res;
    }
    // bellmanFord 負閉路があるなら, dist[s] = INF | O(VE)
    vector<WEIGHT_TYPE> bellmanFord(int s) {
        vector<WEIGHT_TYPE> dist(n, INF);
        dist[s] = 0;
        auto es = getEdge2();
        rep(i, n) {
            for(auto&& e : es) {
                if(dist[e.to] > dist[e.from] + e.weight) {
                    dist[e.to] = dist[e.from] + e.weight;
                    if(i == n-1) {
                        dist[s] = INF;
                        return dist;
                    }
                }
            }
        }
        return dist;
    }
    // bellmanFord s->tの経路上に負閉路があるなら, dist[s] = INF | O(VE)
    vector<WEIGHT_TYPE> bellmanFord2(int s, int t) {
        vector<WEIGHT_TYPE> dist(n, INF);
        auto f1 = getVisitable(s);
        auto f2 = getReverseGraph().getVisitable(t);
        dist[s] = 0;
        auto es = getEdge2();
        rep(i, n) {
            for(auto&& e : es) {
                if(!(f1[e.from]&&f2[e.to])) continue;
                if(dist[e.to] > dist[e.from] + e.weight) {
                    dist[e.to] = dist[e.from] + e.weight;
                    if(i == n-1) {
                        dist[s] = INF;
                        return dist;
                    }
                }
            }
        }
        return dist;
    }
};

int main(){
    int n, m;
    cin >> n >> m;
    GraphD g(n);
    rep(i, m) {
        int a, b;
        cin >> a >> b;
        g.add_edge(a-1, b-1, 1);
    }  
    auto sccs = g.scc();
    for(const auto& scc : sccs) {
        if(scc.size() < 2) continue;
        int k = scc.size();
        vi used(n, 0);
        rep(i, scc.size()) used[scc[i]] = 1;
        rep(i, scc.size()) {
            GraphD h(n);
            for(auto&& e : g.getEdge2()) {
                int u = e.from, v = e.to;
                if(u == scc[i] || v == scc[i]) continue;
                if(used[u] && used[v]) h.add_edge(u, v, 1);
            }
            auto tmp = h.scc();
            rep(j, tmp.size()) {
                if(tmp[j].size() > 1) used[scc[i]] = 0;            
            }
        }
        vector<int> ans;
        rep(i, n) if(used[i]) ans.push_back(i);
        cout << ans.size() << endl;
        rep(i, ans.size()) cout << ans[i]+1 << endl;
        return 0;
    }
    cout << -1 << endl;
    return 0;
}

Measured memory_bytes: 512