Problem:
Score : 500 points

Problem Statement

Ken loves ken-ken-pa (Japanese version of hopscotch). Today, he will play it on a directed graph G.
G consists of N vertices numbered 1 to N, and M edges. The i-th edge points from Vertex u_i to Vertex v_i.

First, Ken stands on Vertex S. He wants to reach Vertex T by repeating ken-ken-pa. In one ken-ken-pa, he does the following exactly three times: follow an edge pointing from the vertex on which he is standing.

Determine if he can reach Vertex T by repeating ken-ken-pa. If the answer is yes, find the minimum number of ken-ken-pa needed to reach Vertex T. Note that visiting Vertex T in the middle of a ken-ken-pa does not count as reaching Vertex T by repeating ken-ken-pa.

Constraints

2 \leq N \leq 10^5

0 \leq M \leq \min(10^5, N (N-1))

1 \leq u_i, v_i \leq N(1 \leq i \leq M)

u_i \neq v_i (1 \leq i \leq M)

If i \neq j, (u_i, v_i) \neq (u_j, v_j).

1 \leq S, T \leq N

S \neq T

Input

Input is given from Standard Input in the following format:

N M
u_1 v_1
:
u_M v_M
S T

Output

If Ken cannot reach Vertex T from Vertex S by repeating ken-ken-pa, print -1.
If he can, print the minimum number of ken-ken-pa needed to reach vertex T.

Sample Input 1

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

Sample Output 1

2

Ken can reach Vertex 3 from Vertex 1 in two ken-ken-pa, as follows: 1 \rightarrow 2 \rightarrow 3 \rightarrow 4 in the first ken-ken-pa, then 4 \rightarrow 1 \rightarrow 2 \rightarrow 3 in the second ken-ken-pa. This is the minimum number of ken-ken-pa needed.

Sample Input 2

3 3
1 2
2 3
3 1
1 2

Sample Output 2

-1

Any number of ken-ken-pa will bring Ken back to Vertex 1, so he cannot reach Vertex 2, though he can pass through it in the middle of a ken-ken-pa.

Sample Input 3

2 0
1 2

Sample Output 3

-1

Vertex S and Vertex T may be disconnected.

Sample Input 4

6 8
1 2
2 3
3 4
4 5
5 1
1 4
1 5
4 6
1 6

Sample Output 4

2

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

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

public class Main {

    public static void main(String[] args) throws IOException {
        Reader in = new Reader();
        HashMap<Integer, ArrayList<Integer>> adj = new HashMap<>();
        int n = in.nextInt();
        int m = in.nextInt();
        for (int i = 0; i < m; i++) {
            int u = in.nextInt();
            int v = in.nextInt();
            if (adj.containsKey(u)) {
                ArrayList<Integer> list = adj.get(u);
                list.add(v);
                adj.put(u, list);
            } else {
                ArrayList<Integer> list = new ArrayList<>();
                list.add(v);
                adj.put(u, list);
            }
        }
        int start = in.nextInt();
        int end = in.nextInt();

        Queue<Node> queue = new LinkedList<>();
        boolean[][] vis = new boolean[n+1][3];
        queue.offer(new Node(start, 0, 0));
        int min = Integer.MAX_VALUE;
        while (!queue.isEmpty()) {
            Node x = queue.poll();
            if (x.id == end) {
                if (x.hops == 0) {
                    min = Math.min(x.dist, min);
                }
            }
            if (adj.containsKey(x.id)) {
                for (int y : adj.get(x.id)) {
                    if (!vis[y][(x.hops+1)%3]) {
                        queue.offer(new Node(y, (x.hops+1)%3, x.dist+1));
                    }
                }
            }
            vis[x.id][x.hops] = true;
        }
        if (min == Integer.MAX_VALUE) {
            System.out.println("-1");
        } else {
            System.out.println(min/3);
        }
    }

    static class Node {
        int id, hops, dist;
        public Node(int id, int hops, int dist) {
            this.id = id;
            this.hops = hops;
            this.dist = dist;
        }
    }

    static class Reader
    {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;

        public Reader()
        {
            din = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public int nextInt() throws IOException
        {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do
            {
                ret = ret * 10 + c - '0';
            }  while ((c = read()) >= '0' && c <= '9');

            if (neg)
                return -ret;
            return ret;
        }

        private void fillBuffer() throws IOException
        {
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }

        private byte read() throws IOException
        {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }

    }
}
/*


 */