Problem:
Score : 600 points

Problem Statement

Given is a directed graph G with N vertices and M edges.

The vertices are numbered 1 to N, and the i-th edge is directed from Vertex A_i to Vertex B_i.

It is guaranteed that the graph contains no self-loops or multiple edges.

Determine whether there exists an induced subgraph (see Notes) of G such that the in-degree and out-degree of every vertex are both 1. If the answer is yes, show one such subgraph.

Here the null graph is not considered as a subgraph.

Notes

For a directed graph G = (V, E), we call a directed graph G' = (V', E') satisfying the following conditions an induced subgraph of G:

V' is a (non-empty) subset of V.

E' is the set of all the edges in E that have both endpoints in V'.

Constraints

1 \leq N \leq 1000

0 \leq M \leq 2000

1 \leq A_i,B_i \leq N

A_i \neq B_i

All pairs (A_i, B_i) are distinct.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
:
A_M B_M

Output

If there is no induced subgraph of G that satisfies the condition, print -1.
Otherwise, print an induced subgraph of G that satisfies the condition, in the following format:

K
v_1
v_2
:
v_K

This represents the induced subgraph of G with K vertices whose vertex set is \{v_1, v_2, \ldots, v_K\}. (The order of v_1, v_2, \ldots, v_K does not matter.)
If there are multiple subgraphs of G that satisfy the condition, printing any of them is accepted.

Sample Input 1

4 5
1 2
2 3
2 4
4 1
4 3

Sample Output 1

3
1
2
4

The induced subgraph of G whose vertex set is \{1, 2, 4\} has the edge set \{(1, 2), (2, 4), (4, 1)\}. The in-degree and out-degree of every vertex in this graph are both 1.

Sample Input 2

4 5
1 2
2 3
2 4
1 4
4 3

Sample Output 2

-1

There is no induced subgraph of G that satisfies the condition.

Sample Input 3

6 9
1 2
2 3
3 4
4 5
5 6
5 1
5 2
6 1
6 2

Sample Output 3

4
2
3
4
5

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

//package com.company;

import java.io.*;
import java.util.*;

public class Main {
    public static class Task {


        public List<Integer> bfsTrackCycle(int n, int m, int start, int end, List<Integer>[] edges) {
            int[] vertexList = new int[m];
            int[] vFrom = new int[m];
            Arrays.fill(vFrom, -1);
            int vFront = 0, vEnd = 0;
            vertexList[vEnd++] = start;
            while (vFront < vEnd) {
                int current = vertexList[vFront++];
                for (int next: edges[current]) {
                    if (vFrom[next] == -1) {
                        vFrom[next] = current;
                        vertexList[vEnd++] = next;
                        if (next == end) {
                            List<Integer> ret = new ArrayList<>();
                            int at = end;
                            while (at != start) {
                                ret.add(at);
                                at = vFrom[at];
                            }
                            ret.add(start);
                            return ret;
                        }
                    }
                }
            }
            return null;
        }
        public void solve(Scanner sc, PrintWriter pw) throws IOException {
            int n = sc.nextInt();
            int m = sc.nextInt();
            List<Integer>[] edges = new List[n];
            for (int i = 0; i < n; i++) {
                edges[i] = new ArrayList<>();
            }
            for (int i = 0; i < m; i++) {
                int a = sc.nextInt() - 1;
                int b = sc.nextInt() - 1;
                edges[a].add(b);
            }
            int bestSize = n + 1;
            List<Integer> ret = null;
            for (int i = 0; i < n; i++) {
                for (int next: edges[i]) {
                    List<Integer> ans = bfsTrackCycle(n, m, next, i, edges);
                    if (ans != null && ans.size() < bestSize) {
                        bestSize = ans.size();
                        ret = ans;
                    }
                }
            }
            if (ret == null) {
                pw.println(-1);
            } else {
                pw.println(ret.size());
                for (int i = 0; i < ret.size(); i++) {
                    pw.println(ret.get(ret.size() - 1 - i) + 1);
                }
            }
        }
    }

    static long TIME_START, TIME_END;

    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(System.in);
//        Scanner sc = new Scanner(new FileInputStream("input"));
        PrintWriter pw = new PrintWriter(new BufferedOutputStream(System.out));
//        PrintWriter pw = new PrintWriter(new FileOutputStream("output"));

        Runtime runtime = Runtime.getRuntime();
        long usedMemoryBefore = runtime.totalMemory() - runtime.freeMemory();
        TIME_START = System.currentTimeMillis();
        Task t = new Task();
        t.solve(sc, pw);
        TIME_END = System.currentTimeMillis();
        long usedMemoryAfter = runtime.totalMemory() - runtime.freeMemory();
        pw.close();
        System.err.println("Memory increased: " + (usedMemoryAfter - usedMemoryBefore) / 1000000);
        System.err.println("Time used: " + (TIME_END - TIME_START) + ".");
    }

    static class Scanner {
        StringTokenizer st;
        BufferedReader br;

        public Scanner(InputStream s) {
            br = new BufferedReader(new InputStreamReader(s));
        }

        public Scanner(FileReader s) throws FileNotFoundException {
            br = new BufferedReader(s);
        }

        public String next() throws IOException {
            while (st == null || !st.hasMoreTokens())
                st = new StringTokenizer(br.readLine());
            return st.nextToken();
        }

        public int nextInt() throws IOException {
            return Integer.parseInt(next());
        }

        public long nextLong() throws IOException {
            return Long.parseLong(next());
        }

        public String nextLine() throws IOException {
            return br.readLine();
        }

        public double nextDouble() throws IOException {
            return Double.parseDouble(next());
        }

        public boolean ready() throws IOException {
            return br.ready();
        }
    }
}