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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.InputMismatchException;
import java.util.HashSet;
import java.io.FileInputStream;
import java.io.PrintWriter;
import java.util.Queue;
import java.io.IOException;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.Set;
import java.io.FileNotFoundException;
import java.util.List;
import java.util.ArrayDeque;
import java.io.InputStream;

public class Main {
    InputStream is;

    int __t__ = 1;
    int __f__ = 0;
    int __FILE_DEBUG_FLAG__ = __f__;
    String __DEBUG_FILE_NAME__ = "src/F7";

    FastScanner in;
    PrintWriter out;

    Graph<SimpleEdge> g;
    public void solve() {
        int n = in.nextInt(), m = in.nextInt();
        g = Graph.create(n);

        for (int i = 0; i < m; i++) {
            int a = in.nextInt() - 1;
            int b = in.nextInt() - 1;
            g.addDirectionalEdge(a, new SimpleEdge(b));
        }

        int[] depth = new int[n];
        Set<Integer> cur = null;
        for (int i = 0; i < n; i++) {
            Arrays.fill(depth, -1);
            Set<Integer> nodes = new HashSet<>();
            if (dfs(i, depth, cur, nodes, 0) != -1) {
                cur = nodes;
                break;
            }
        }
        if (cur == null) {
            System.out.println("-1");
            return;
        }

        while (true) {
            Arrays.fill(depth, -1);
            Set<Integer> nodes = new HashSet<>();

            int start = -1;
            for (int i = 0; i < n; i++) {
                if (cur.contains(i)) {
                    start = i;
                    break;
                }
            }
            int goal = dfs(start, depth, cur, nodes, 0);
            if (goal == -1) {
                break;
            }
            cur = nodes;
        }

        System.out.println(cur.size());
        for (int x : cur) {
            System.out.println(x + 1);
        }
    }

    private int dfs(int u, int[] depth, Set<Integer> cur, Set<Integer> res, int curDep) {
        depth[u] = curDep;
        for (Edge e : g.edges(u)) {
            if (cur != null && !cur.contains(e.to())) {
                continue;
            }
            if (depth[e.to()] != -1) {
                if (cur == null || curDep - depth[e.to()] + 1 < cur.size()) {
                    res.add(u);
                    return e.to();
                }
            } else {
                int goal = dfs(e.to(), depth, cur, res, curDep + 1);
                if (goal == -2) {
                    return -2;
                } else if (goal != -1) {
                    res.add(u);
                    return (goal == u) ? -2 : goal;
                }
            }
        }
        depth[u] = -1;
        return -1;
    }

    public void run() {
        if (__FILE_DEBUG_FLAG__ == __t__) {
            try {
                is = new FileInputStream(__DEBUG_FILE_NAME__);
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
            System.out.println("FILE_INPUT!");
        } else {
            is = System.in;
        }
        in = new FastScanner(is);
        out = new PrintWriter(System.out);

        solve();
    }

    public static void main(final String[] args) {
        new Main().run();
    }
}

class SimpleEdge implements Edge {
    private final int to;

    public SimpleEdge(int to) {
        this.to = to;
    }

    @Override
    public int to() {
        return to;
    }

    @Override
    public long cost() {
        return 1;
    }
}


class Graph<E extends Edge> {
    private final int n;
    private final List<E>[] g;

    public static <E extends Edge> Graph<E> create(int n) {
        @SuppressWarnings("unchecked")
        ArrayList<E>[] l = new ArrayList[n];
        for (int i = 0; i < n; i++) {
            l[i] = new ArrayList<>();
        }
        return new Graph<>(n, l);
    }

    public Graph(int n, List<E>[] g) {
        this.n = n;
        this.g = g;
    }

    public static Graph<SimpleEdge> createDirectionalTree(int[] a, int[] b) {
        if (a.length != b.length) {
            throw new IllegalArgumentException("len(a) != len(b)");
        }
        int n = a.length + 1;
        Graph<SimpleEdge> g = create(n);
        for (int i = 0; i < n - 1; i++) {
            g.addDirectionalEdge(a[i], new SimpleEdge(b[i]));
        }
        return g;
    }

    public static Graph<SimpleEdge> createUndirectionalTree(int[] a, int[] b, boolean oneOrigin) {
        if (a.length != b.length) {
            throw new IllegalArgumentException("len(a) != len(b)");
        }
        int n = a.length + 1;
        Graph<SimpleEdge> g = create(n);

        int adjust = oneOrigin ? 1 : 0;
        for (int i = 0; i < n - 1; i++) {
            g.addDirectionalEdge(a[i] - adjust, new SimpleEdge(b[i] - adjust));
            g.addDirectionalEdge(b[i] - adjust, new SimpleEdge(a[i] - adjust));
        }
        return g;
    }

    public void addDirectionalEdge(int u, E e) {
        g[u].add(e);
    }

    public List<E> edges(int u) {
        return g[u];
    }

    public int edgeNum(int u) {
        return g[u].size();
    }

    public int totalEndgeNum() {
        return Arrays.stream(g)
                .map(List::size)
                .reduce(0, Integer::sum) / 2;
    }

    public int size() {
        return n;
    }

    /**
     *
     * @param start
     * @return The cost of node v is:
     * <ul>
     *     <li>-(1L << 62) if a node is under negative loop</li>
     *     <li>(1L << 62) if there is no edge to v</li>
     *     <li>otherwise minimum cost from node u to a node</li>
     * </ul>
     */
    public long[] bellmanford(int start) {
        final long INF = 1L << 62;
        long[] dist = new long[n];
        Arrays.fill(dist, INF);
        dist[start] = 0;
        for (int i = 0; i < n - 1; i++) {
            for (int u = 0; u < n; u++) {
                for (Edge e : edges(u)) {
                    dist[e.to()] = Math.min(dist[e.to()], dist[u] + e.cost());
                }
            }
        }

        long[] res = Arrays.copyOf(dist, dist.length);
        for (int u = 0; u < n; u++) {
            for (Edge e : edges(u)) {
                int v = e.to();
                if (dist[u] + e.cost() < dist[v]) {
                    res[v] = -INF;
                }
            }
        }
        return res;
    }

    /**
     *
     * @param start
     * @return For each v, true if a node v can be arrived from start node, otherwise false.
     */
    public boolean[] canArrive(int start) {
        boolean[] res = new boolean[size()];
        canArriveInternal(start, res);
        return res;
    }

    private void canArriveInternal(int u, boolean[] visited) {
        visited[u] = true;
        for (Edge e : edges(u)) {
            if (visited[e.to()]) {
                continue;
            }
            canArriveInternal(e.to(), visited);
        }
    }

    public DiameterResult diameter() {
        if (totalEndgeNum() != n - 1) {
            throw new IllegalArgumentException("Graph should be tree");
        }
        int[] parents = new int[size()];
        int[] dist = new int[size()];
        Arrays.fill(dist, -1);
        dfs(0, dist, parents, 0);

        int x = 0;
        for (int i = 1; i < size(); i++) {
            if (dist[x] < dist[i]) {
                x = i;
            }
        }

        int[] dist2 = new int[size()];
        Arrays.fill(dist2, -1);
        parents[x] = x;
        dfs(x, dist2, parents, 0);

        int best = 0;
        for (int i = 0; i < n; i++) {
            if (dist2[best] < dist2[i]) {
                best = i;
            }
        }
        return new DiameterResult(restore(x, best, parents), dist2[best] + 1);
    }

    private int[] restore(int from, int to, int[] parents) {
        List<Integer> pathL = new ArrayList<>();
        int cur = to;
        while (cur != from) {
            pathL.add(cur);
            cur = parents[cur];
        }
        pathL.add(from);

        int[] res = new int[pathL.size()];
        for (int i = 0; i < pathL.size(); i++) {
            res[i] = pathL.get(pathL.size() - i - 1);
        }
        return res;
    }

    private void dfs(int u, int[] dist, int[] parent, int depth) {
        dist[u] = depth;
        for (Edge e : g[u]) {
            if (dist[e.to()] != -1) {
                continue;
            }
            parent[e.to()] = u;
            dfs(e.to(), dist, parent, depth + 1);
        }
    }

    public static class DiameterResult {
        public final int[] path;
        public final int dist;

        public DiameterResult(int[] path, int dist) {
            this.path = path;
            this.dist = dist;
        }

        @Override
        public String toString() {
            return "DiameterResult{" +
                    "path=" + Arrays.toString(path) +
                    ", dist=" + dist +
                    '}';
        }
    }

    /**
     * Assuming edge is SimpleEdge
     * @param start
     * @return
     */
    public int[] bfs(int start) {
        int n = g.length;
        int[] dist = new int[n];
        Arrays.fill(dist, 1 << 30);
        dist[start] = 0;

        Queue<Integer> q = new ArrayDeque<>();
        q.add(start);
        while (!q.isEmpty()) {
            int u = q.poll();
            for (Edge e : g[u]) {
                if (dist[e.to()] == 1 << 30) {
                    dist[e.to()] = dist[u] + 1;
                    q.add(e.to());
                }
            }
        }
        return dist;
    }
}

class EdgeImpl implements Edge {
    private final int to;
    private final long cost;

    public EdgeImpl(int to, long cost) {
        this.to = to;
        this.cost = cost;
    }

    @Override
    public int to() {
        return to;
    }

    @Override
    public long cost() {
        return cost;
    }
}

interface EdgeGenerator<E extends Edge> {
    E generate(int to, E originalEdge);
}

interface Edge {
    int to();

    long cost();
}


class FastScanner {
    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;

    public FastScanner(InputStream stream) {
        this.stream = stream;
        // stream = new FileInputStream(new File("dec.in"));
    }

    int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    public boolean isEndline(int c) {
        return c == '\n' || c == '\r' || c == -1;
    }

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

    public int[] nextIntArray(int n) {
        int[] array = new int[n];
        for (int i = 0; i < n; i++)
            array[i] = nextInt();

        return array;
    }

    public int[][] nextIntMap(int n, int m) {
        int[][] map = new int[n][m];
        for (int i = 0; i < n; i++) {
            map[i] = nextIntArray(m);
        }
        return map;
    }

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

    public long[] nextLongArray(int n) {
        long[] array = new long[n];
        for (int i = 0; i < n; i++)
            array[i] = nextLong();

        return array;
    }

    public long[][] nextLongMap(int n, int m) {
        long[][] map = new long[n][m];
        for (int i = 0; i < n; i++) {
            map[i] = nextLongArray(m);
        }
        return map;
    }

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

    public double[] nextDoubleArray(int n) {
        double[] array = new double[n];
        for (int i = 0; i < n; i++)
            array[i] = nextDouble();

        return array;
    }

    public double[][] nextDoubleMap(int n, int m) {
        double[][] map = new double[n][m];
        for (int i = 0; i < n; i++) {
            map[i] = nextDoubleArray(m);
        }
        return map;
    }

    public String next() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public String[] nextStringArray(int n) {
        String[] array = new String[n];
        for (int i = 0; i < n; i++)
            array[i] = next();

        return array;
    }

    public String nextLine() {
        int c = read();
        while (isEndline(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isEndline(c));
        return res.toString();
    }

    public int[][] nextPackedIntArrays(int packN, int size) {
        int[][] res = new int[packN][size];
        for (int i = 0; i < size; i++) {
            for (int j = 0; j < packN; j++) {
                res[j][i] = nextInt();
            }
        }
        return res;
    }
}