//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <fstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>
#include <chrono>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

constexpr int MAX_V = 100010;
constexpr int INF = 1e8;

vector<int> G[MAX_V];

typedef pair<int, int> TT;

void dijkstra(int S, int N, int M, vector<vector<int>> &dist) {
    dist[0][S] = 0;
    priority_queue<TT, vector<TT>, greater<>> Q;
    Q.push(make_pair(0, S));
    while (!Q.empty()) {
        auto p = Q.top();
        Q.pop();
        int cost = p.first;
        int v = p.second;
        if (dist[cost % 3][v] < cost) continue;
        for (int i = 0; i < G[v].size(); i++) {
            int to = G[v][i];
            int toCost = cost + 1;
            int toState = toCost % 3;
            if(dist[toState][to] > toCost){
                dist[toState][to] = toCost;
                Q.push(make_pair(toCost, to));
            }
        }
    }
}

int main() {

    int N, M;
    cin >> N >> M;

    for (int i = 0; i < M; i++) {
        int a, b;
        cin >> a >> b;
        a--, b--;
        G[a].push_back(b);
    }

    int S, T;
    cin >> S >> T, S--, T--;

    vector<vector<int>> dist(3, vector<int>(MAX_V, INF));

    dijkstra(S, N, M, dist);

    if(dist[0][T] == INF){
        cout << -1 << endl;
        return 0;
    }else{
        cout << dist[0][T] /3 << endl;
    }

}






































Measured memory_bytes: 7296