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

typedef long long ll;
typedef pair<int, int> pii;
const int N = 1e5 + 100, M = 3, inF = 1e9 + 100;

int n, m, s, t, dis[N][M];
vector<int> nei[N];
queue<pii> q;

void bfs(pii p) {
	int v = p.first, t = p.second;
	for (int u: nei[v]) {
		int nxt_v = u, nxt_t = (t + 1) % 3;
		if(dis[nxt_v][nxt_t] == inF) {
			dis[nxt_v][nxt_t] = dis[v][t] + 1;
			q.push(pii(nxt_v, nxt_t));
		}
	}
}

int main() {
	ios::sync_with_stdio(false), cin.tie(0);
	for (int i = 0; i < N; i++)
		fill(dis[i], dis[i] + M, inF);
	cin >> n >> m;
	while(m--) {
		int u, v;
		cin >> u >> v;
		nei[--u].push_back(--v);
	}
	cin >> s >> t;
	s--;
	t--;
	dis[s][0] = 0;
	q.push(pii(s, 0));
	while(q.size()) {
		bfs(q.front());
		q.pop();
	}
	cout << (dis[t][0] == inF? -1: dis[t][0] / 3);
	return 0;
}