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 C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

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

using int32 = int_fast32_t;
using int64 = int_fast64_t;

const int32 INF = 1e9;
const int32 MOD = 1e9+7;
const int64 LLINF = 1e18;

#define YES(n) cout << ((n) ? "YES\n" : "NO\n"  )
#define Yes(n) cout << ((n) ? "Yes\n" : "No\n"  )
#define ANS(n) cout << (n) << "\n"
#define REP(i,n) for(int64 i=0;i<(n);++i)
#define FOR(i,a,b) for(int64 i=(a);i<(b);i++)
#define FORR(i,a,b) for(int64 i=(a);i>=(b);i--)
#define ALL(obj) (obj).begin(),(obj).end()
#define pii pair<int32,int32>
#define pll pair<int64,int64>
#define pb(a) push_back(a)
#define mp make_pair

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int32 n,m;
    cin >> n >> m;
    vector<vector<int32>> adjacent(n);
    REP(i,m){
        int32 u,v;
        cin >> u >> v;
        u--;v--;
        adjacent[u].pb(v);
    }
    int32 s,t;
    cin >> s >> t;
    s--;t--;
    vector<vector<bool>> canBeReached(3, vector<bool>(n, false));
    canBeReached[0][s] = true;
    queue<int32> que[3];
    que[0].push(0);
    for(int64 i = 0;; ++i){
        REP(j,3){
            bool flg = (j == 2);
            REP(a,que[j].size()){
                int32 k = que[j].front();que[j].pop();
                if(canBeReached[j][k]){
                    REP(l,adjacent[k].size()){
                        if(!canBeReached[(j+1)%3][adjacent[k][l]]){
                            flg = false;
                            canBeReached[(j+1)%3][adjacent[k][l]] = true;
                            que[(j+1)%3].push(l);
                        }
                    }
                }
            }
            if(flg){
                ANS(-1);
                return 0;
            }
        }
        if(canBeReached[0][t]){
            ANS(i+1);
            return 0;
        }
    }
    return 0;
}