#include "bits/stdc++.h"
using namespace std;
using ll = long long;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vvvll = std::vector<vvll>;
using dd = double;
using vdd = std::vector<dd>;
using vvdd = std::vector<vdd>;
using vvvdd = std::vector<vvdd>;

const ll INF = 1LL << 45; //10e5倍してもオーバーフローしない値
const ll MOD = 1000000007LL;// 998244353L;
const dd EPS = 1e-11;

typedef std::numeric_limits< double > dbl;

//cin,cout高速化のおまじない＋桁数指定
struct Fast{
	Fast(){
		cin.tie(0);
		ios::sync_with_stdio(false);
		cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
	}
} fast;

#define REPS(i, S, E) for (ll i = (S); i <= (E); i++)
#define REP(i, N) REPS(i, 0, N-1)
#define DEPS(i, E, S) for (ll i = (E); i >= (S); i--)
#define DEP(i, N) DEPS(i, N-1, 0)
#define EACH(e, v) for (auto&& e : v)

template<class T> inline bool chmax(T &a, T b) { if (a < b) { a = b; return true; }return false; }
template<class T> inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; }return false; }
template<class T> inline T MaxE(vector<T> &v, ll S, ll E){ T m=v[S]; REPS(i, S, E)chmax(m, v[i]); return m; }//v[S]～v[E]の最大値
template<class T> inline T MinE(vector<T> &v, ll S, ll E){ T m=v[S]; REPS(i, S, E)chmin(m, v[i]); return m; }//v[S]～v[E]の最小値
template<class T> inline T MaxE(vector<T> &v, ll N) { return MaxE(v, 0, N - 1); } //先頭N個中の最大値
template<class T> inline T MinE(vector<T> &v, ll N) { return MinE(v, 0, N - 1); }
template<class T> inline T MaxE(vector<T> &v) { return MaxE(v, (ll)v.size()); }
template<class T> inline T MinE(vector<T> &v) { return MinE(v, (ll)v.size()); }
template<class T> inline ll MaxI(vector<T> &v, ll S, ll E){ ll m=S; REPS(i, S, E){ if (v[i]>v[m])m=i; }return m; }
template<class T> inline ll MinI(vector<T> &v, ll S, ll E){ ll m=S; REPS(i, S, E){ if (v[i]<v[m])m=i; }return m; }
template<class T> inline ll MaxI(vector<T> &v, ll N) { return MaxI(v, 0, N - 1); }
template<class T> inline ll MinI(vector<T> &v, ll N) { return MinI(v, 0, N - 1); }
template<class T> inline ll MaxI(vector<T> &v) { return MaxI(v, (ll)v.size()); }
template<class T> inline ll MinI(vector<T> &v) { return MinI(v, (ll)v.size()); }
template<class T> inline T Sum(vector<T> &v, ll S, ll E){ T s=v[S]; REPS(i, S+1, E)s+=v[i]; return s; }
template<class T> inline T Sum(vector<T> &v, ll N) { return Sum(v, 0, N - 1); }
template<class T> inline T Sum(vector<T> &v) { return Sum(v, v.size()); }
template<class T> inline T SumMOD(vector<T> &v, ll S, ll E){ T s=v[S]; REPS(i, S+1, E)s=(s+v[i])%MOD; return s; }
template<class T> inline T SumMOD(vector<T> &v, ll N) { return SumMOD(v, 0, N - 1); }
template<class T> inline T SumMOD(vector<T> &v) { return SumMOD(v, v.size()); }

inline ll AddMOD(ll a, ll b) { return (a + b) % MOD; }
inline ll SubMOD(ll a, ll b) { return (a - b + MOD) % MOD; }
inline ll MulMOD(ll a, ll b) { return (a * b) % MOD; }

inline ll in() { ll x; cin >> x; return x; }
inline double inD() { double x; cin >> x; return x; }
inline string inS() { string x; cin >> x; return std::move(x); }
template<class T=ll> inline vector<T> in_v(ll N){ vector<T> v(N); REP(i, N)cin>>v[i]; return move(v); }
inline void To0origin(vll &v) { EACH(e, v) e--; }

vvll in_Graph(ll nodeNum, ll edgeNum, bool isDirected){
	vvll to(nodeNum);
	REP(i, edgeNum){
		ll v, u;
		cin >> v >> u;
		v--; u--;
		to[v].push_back(u);
		if (!isDirected) to[u].push_back(v);
	}
	return std::move(to);
}


ll N, M, S, T;
vvll to;
vll dist;

int idx_(int v, int state){ return v*3+(state%3); }
int v_(int idx){ return idx/3; }
int state_(int idx){ return idx%3; }

ll &Dist(int v, int state){ return dist[idx_(v, state)]; }

void bfs(){
	queue<ll> que;
	
	Dist(S, 0) = 0;
	que.push(idx_(S, 0));

	while (!que.empty()){
		int idx = que.front();  que.pop();
		int v = v_(idx);
		int state = state_(idx);

		EACH(u, to[v]){
			if (Dist(u, state+1) != -1) continue;
			Dist(u, state+1) = Dist(v, state) + 1;
			que.push(idx_(u, state+1));

			if (idx_(u, state+1) == idx_(T, 0)) return;
		}
	}
}

int main()
{
	cin >> N >> M;
	to = in_Graph(N, M, true);
	cin >> S >> T;
	S--; T--;

	dist.resize(N*3, -1);
	bfs();

	ll ans = Dist(T, 0);
	if (ans != -1) ans /= 3;
	cout << ans << '\n';
	return 0;
}


Measured memory_bytes: 8064