#include <iostream>
#include <map>
#include <set>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <fstream>
#include <bitset>
#include <queue>
#include <stack>
#include <deque>
#include <time.h>
#include <complex>

using std::cin;
using std::cout;
using std::cerr;
using std::endl;

using std::map;
using std::set;
using std::bitset;
using std::vector;
using std::string;
using std::multimap;
using std::multiset;
using std::deque;
using std::queue;
using std::stack;
using std::pair;
using std::iterator;
using std::complex;

using std::sort;
using std::stable_sort;
using std::reverse;
using std::max_element;
using std::min_element;
using std::unique;
using std::ios_base;
using std::swap;
using std::min;
using std::max;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef string S;
typedef complex<double> cd;

namespace MySpace{
    struct Small_Segment_Tree{
        vector<int> a;
        int l, r;

        private:
        int __summ(int L, int R, int l, int r, int v)
        {
            if (L <= l && r <= R)
                return a[v];
            if (R <= l || r <= L)
                return 0;
            int m = (l + r) / 2;
            return __summ(L, R, l, m, 2 * v + 1) + __summ(L, R, m, r, 2 * v + 2);
        }

        void __set(int P, int l, int r, int v, int V)
        {
            if (l + 1 == r)
            {
                a[v] = V;
                return;
            }
            int m = (l + r) / 2;
            if (P < m)
            {
                __set(P, l, m, 2 * v + 1, V);
                a[v] = a[2 * v + 1] + a[2 * v + 2];
            }
            else
            {
                __set(P, m, r, 2 * v + 2, V);
                a[v] = a[2 * v + 1] + a[2 * v + 2];
            }
        }

        public:
        int Summ(int L, int R)
        {
            if (R < L)
                return 0;
            return __summ(L, R + 1, l, r, 0);
        }

        void Set(int P, int V)
        {
            __set(P, l, r, 0, V);
        }

        Small_Segment_Tree(int _n)
        {
            a.resize(4 * _n);
            l = 0, r = _n;
        }
    };
    long long inq(long long x, long long q, long long MOD)
    {
        if (q == 0)
            return 1;
        long long l = inq(x, q / 2, MOD);
        if (q % 2)
            return l * l % MOD * x % MOD;
        return l * l % MOD;
    }
};


using namespace MySpace;

#define F(i, n) for (int (i) = 0; (i) != (n); (i)++)
#define fi first
#define se second
#define re return
#define all(x) (x).begin(), (x).end()

const ll INF = 1e18;
ll n, m;
vector<pair<int, int> > graph[3][500000];
ll dist[3][500000];

signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    ll s, t;
    cin >> n >> m;
    for (int i = 0; i < m; i++)
    {
        int u, v;
        cin >> v >> u;
        graph[0][v].push_back({1, u});
        graph[1][v].push_back({2, u});
        graph[2][v].push_back({0, u});
    }
    cin >> s >> t;
    vector<pair<int, int> > q;
    q.push_back({0, s});
    for (int i = 0; i <= n; i++)
    {
        dist[0][i] = -3;
        dist[1][i] = -3;
        dist[2][i] = -3;
    }
    dist[0][s] = 0;
    for (int i = 0; i < q.size(); i++)
    {
        int a1 = q[i].first;
        int a2 = q[i].second;
        for (int j = 0; j < graph[a1][a2].size(); j++)
        {
            int b1 = graph[a1][a2][j].first;
            int b2 = graph[a1][a2][j].second;
            if (dist[b1][b2] == -3)
            {
                dist[b1][b2] = dist[a1][a2] + 1;
                q.push_back({b1, b2});
            }
        }
    }
    cout << dist[0][t] / 3;
}


Measured memory_bytes: 61552