#include <bits/stdc++.h>
using namespace std;

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> P;
typedef tuple<int,int,int> tpl;

#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define SORT(c) sort((c).begin(),(c).end())
#define REVERSE(c) reverse((c).begin(),(c).end())
#define LB(a,x) lower_bound((a).begin(), (a).end(), x) - (a).begin()
#define UB(a,x) upper_bound((a).begin(), (a).end(), x) - (a).begin()

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define RFOR(i,a,b) for(int i=(a)-1;i>=(b);--i)
#define RREP(i,n) RFOR(i,n,0)

#define en "\n"

const double EPS = 1e-9;
const double PI  = acos(-1.0);
const int INT_INF = 2147483647;
const long long LL_INF = 1LL<<60;
const long long MOD = 1000000007; // 998244353

#define CLR(a) memset((a), 0, sizeof(a))

#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

struct Node{
    int id, v;
    bool done = false;
    Node(int id, int v=1e9): id(id), v(v) {}
    Node(): id(0), v(1e9) {}
};

bool operator>(Node a, Node  b){
    return a.v > b.v;
}

struct Edge{
    int from, to, cost, id;
    Edge(int from, int to, int cost=1, int id=-1): from(from), to(to), cost(cost), id(id) {}
};

struct Graph{
    int N;
    vector<vector<Edge>> adj;
    vector<Node> node;

    Graph(int N): N(N) {
        adj.assign(N, vector<Edge>());
        node.assign(N, Node());
        for(int i=0; i<N; ++i) node[i].id = i;
    }

    Graph() {}

    void add_edge(int i, int j, int cost=1, int id=-1){
        Edge e(i, j, cost, id);
        adj[i].emplace_back(e);
    }

    vector<Edge> get_adj(Node node){
        return adj[node.id];
    }

    void assign(int n){
        N = n;
        adj.assign(N, vector<Edge>());
        node.assign(N, Node());
        for(int i=0; i<N; ++i) node[i].id = i;
    }

    void reset(){
        for(int i=0; i<N; ++i){
            node[i].done = false;
            node[i].v = 1e9;
        }
    }
};

int N,M,fin = 0;
VI uv, nxt, calced;
Graph G;

bool dfs(int n){
    if(G.node[n].done && calced[n] == 0){
        uv[n] = 1;
        return true;
    }
    G.node[n].done = true;
    for(Edge e : G.adj[n]){
        if(dfs(e.to)){
            if(uv[n] == 0 && fin == 0){
                uv[n] = 1;
                nxt[n] = e.to;
            }
            else if(uv[n] == 1 && fin == 0){
                fin = 1;
                nxt[n] = e.to;
            }
            return true;
        }
    }
    calced[n] = 1;
    return false;
}
int main(void){
    cin >> N >> M;
    G.assign(N); uv.assign(N,0); nxt.assign(N,-1); calced.assign(N,0);
    REP(i,M){
        int a,b; cin >> a >> b; a--; b--;
        G.add_edge(a,b,1,i);
    }
    bool flag = true;
    REP(i,N){
        if(G.node[i].done) continue;
        if(dfs(i)){
            flag = false;
            break;
        }
    }
    if(flag){
        cout << -1 << endl;
        return 0;
    }
    while(1){
        VI newv(N,0);
        flag = true;
        int s, g;
        REP(i,N){
            if(uv[i] == 0) continue;
            bool flag2 = false;
            for(Edge e : G.adj[i]){
                if(uv[e.to] == 1 && nxt[i] != e.to){
                    s = e.to;
                    g = i;
                    nxt[i] = e.to;
                    flag2 = true;
                    break;
                }
            }
            if(flag2){
                flag = false;
                break;
            }
        }
        if(flag){
            VI ans;
            REP(i,N) if(uv[i]) ans.push_back(i+1);
            cout << ans.size() << en;
            REP(i,ans.size()) cout << ans[i] << en;
            return 0;
        }

        newv[g] = 1;
        while(s != g){
            newv[s] = 1;
            s = nxt[s];
        }
        swap(uv,newv);
    }
    return 0;
}

Measured memory_bytes: 384