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: Wrong Answer)

import java.io.*;
import java.util.*;
 
public class Main {
	public static int mindist = Integer.MAX_VALUE;
	public static ArrayList graph[];
	public static int vis[];
	public static void kenpa(int node)
	{
		if(vis[node]==1)
			return;
		vis[node] = 1;
		ArrayList<Integer> list = (ArrayList<Integer>)graph[node];
		ArrayList<Integer> dist = new ArrayList<Integer>();
		ArrayList<Integer> temp;
		int len = list.size();
		int next;
		for(int i=0;i<len;i++)
		{
			next = list.get(i);
			kenpa(next);
			temp = graph[next];
			for(int j=0;j<temp.size();j++)
			{
				dist.add(temp.get(j)+1);
			}
		}
		graph[node] = dist;
	}
	public static void main(String[] args)throws IOException {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);
        
		int n = in.nextInt();
		int m = in.nextInt();
		ArrayList adjacency[] = new ArrayList[n+1];
		int one,two;
		for(int x=0;x<n+1;x++)
		{
			adjacency[x] = new ArrayList<Integer>();
		}
		for(int i=0;i<m;i++)
		{
			one = in.nextInt();
			two = in.nextInt();
			adjacency[one].add(two);
		}
		int source = in.nextInt();
		int dest = in.nextInt();
		int visited[] = new int[n+1];
		Arrays.fill(visited,0);
		adjacency[dest].add(0);
		visited[dest]=1;
		vis=visited;
		graph = adjacency;
		kenpa(source);
		ArrayList<Integer> sourcelist = (ArrayList<Integer>)graph[source];
		int d;
		for(int i=0;i<sourcelist.size();i++)
		{
			d = sourcelist.get(i);
			if(d%3==0)
				mindist = Math.min((int)(d/3),mindist);
		}
		if(mindist==Integer.MAX_VALUE)
			out.println(-1);
		else
			out.println(mindist);
        out.close();
	}
    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;
 
        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }
 
        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
 
        public int nextInt() {
            return Integer.parseInt(next());
        }
 
        public long nextLong() {
            return Long.parseLong(next());
        }
 
    }
 
}