//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>

#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

#include <stdexcept>

using namespace std;

//conversion
//------------------------------------------
inline int toint(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class t> inline string tostring(t x) { ostringstream sout; sout << x; return sout.str(); }

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl
#define debug(x) cerr << #x << " = " << (x) << " (l" << __line__ << ")" << " " << __file__ << endl

// type alias
using ll = long long;
using ull = unsigned long long;

inline ll floor(ll a, ull b)
{
	ll llb = b;
	if (a >= 0)
	{
		return a / llb;
	}
	else
	{
		return - ( (-a + llb - 1) / llb);
	}
}

inline ll ceil(ll a, ull b)
{
	ll llb = b;
	if (a >= 0)
	{
		return (a + llb - 1) / llb;
	}
	else
	{
		return - (-a / llb);
	}
}

inline bool isPower2(ull a)
{
	if (a == 0)
	{
		return false;
	}
	return !(a & (a - 1));
}

inline ll getMaxEffectiveBit(ull a)
{
	ll index = -1;
	for (; a != 0; a >>= 1)
	{
		++index;
	}
	return index;
}

inline ll getSpecifiedBit(ull a, unsigned bit)
{
	return (a & (1LL << bit)) >> bit;
}

// computational complexity: o(log(max(a, b))) 
inline ull getGcd(ull a, ull b)
{
	if (b == 0)
	{
		return a;
	}
	return getGcd(b, a % b);
}

template<class Integer>
inline Integer getPower(Integer base, ull exponential)
{
	Integer result = 1;
	while (exponential >= 1)
	{
		if (exponential & 1)
		{
			result = result * base;
		}
		base = base * base;
		exponential >>= 1;
	}

	return result;
}

template <class Integer>
class ResidueInteger
{
private:
	Integer mod_;
	Integer n_;
	static Integer global_mod_;

	// computational complexity: o(log(max(a, b))) 
	static inline pair<Integer, Integer> getBezoutsIdentitySolution(Integer a, Integer b)
	{
		if (b == 0)
		{
			return { 1, 0 };
		}
		auto sol = getBezoutsIdentitySolution(b, a % b);
		return { sol.second, sol.first - (a / b) * sol.second };
	}

public:
	Integer getModValue(const Integer& n) const
	{
		return (n % mod_ + mod_) % mod_;
	}

	// computational complexity: o(log(max(n, mod))) 
	inline Integer getModInverse(const Integer& n) const
	{
		auto sol = getBezoutsIdentitySolution(n, mod_);
		if (n * sol.first + mod_ * sol.second != 1)
		{
			return -1;
		}
		return getModValue(sol.first);
	}

	ResidueInteger() : mod_(global_mod_), n_(getModValue(0)) {}
	ResidueInteger(Integer n) : mod_(global_mod_), n_(getModValue(n)) {}
	ResidueInteger(Integer n, Integer mod) : mod_(mod), n_(getModValue(n)) {}

	inline ResidueInteger operator+(const ResidueInteger& rhs) const
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		return { this->n_ + rhs.n_, this->mod_ };
	}
	inline ResidueInteger operator+(const Integer& rhs) const
	{
		return { this->n_ + getModValue(rhs), this->mod_ };
	}
	friend inline ResidueInteger operator+(const Integer& lhs, const ResidueInteger& rhs)
	{
		return { rhs.getModValue(lhs) + rhs.n_, rhs.mod_ };
	}
	inline ResidueInteger& operator+=(const ResidueInteger& rhs)
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		this->n_ = getModValue(this->n_ + rhs.n_);
		return *this;
	}
	inline ResidueInteger& operator+=(const Integer& rhs)
	{
		this->n_ = getModValue(this->n_ + getModValue(rhs));
		return *this;
	}

	inline ResidueInteger operator-(const ResidueInteger& rhs) const
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		return { this->n_ - rhs.n_, this->mod_ };
	}
	inline ResidueInteger operator-(const Integer& rhs) const
	{
		return { this->n_ - getModValue(rhs), this->mod_ };
	}
	friend inline ResidueInteger operator-(const Integer& lhs, const ResidueInteger& rhs)
	{
		return { rhs.getModValue(lhs) - rhs.n_, rhs.mod_ };
	}
	inline ResidueInteger& operator-=(const ResidueInteger& rhs)
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		this->n_ = getModValue(this->n_ - rhs.n_);
		return *this;
	}
	inline ResidueInteger& operator-=(const Integer& rhs)
	{
		this->n_ = getModValue(this->n_ - getModValue(rhs));
		return *this;
	}

	inline ResidueInteger operator*(const ResidueInteger& rhs) const
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		return { this->n_ * rhs.n_, this->mod_ };
	}
	inline ResidueInteger operator*(const Integer& rhs) const
	{
		return { this->n_ * getModValue(rhs), this->mod_ };
	}
	friend inline ResidueInteger operator*(const Integer& lhs, const ResidueInteger& rhs)
	{
		return { rhs.getModValue(lhs) * rhs.n_, rhs.mod_ };
	}
	inline ResidueInteger& operator*=(const ResidueInteger& rhs)
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		this->n_ = getModValue(this->n_ * rhs.n_);
		return *this;
	}
	inline ResidueInteger& operator*=(const Integer& rhs)
	{
		this->n_ = getModValue(this->n_ * getModValue(rhs));
		return *this;
	}

	inline ResidueInteger operator/(const ResidueInteger& rhs) const
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		return { this->n_ * getModInverse(rhs.n_), this->mod_ };
	}
	inline ResidueInteger operator/(const Integer& rhs) const
	{
		return { this->n_ * getModInverse(getModValue(rhs)), this->mod_ };
	}
	friend inline ResidueInteger operator/(const Integer& lhs, const ResidueInteger& rhs)
	{
		return { rhs.getModValue(lhs) * rhs.getModInverse(rhs.n_), rhs.mod_ };
	}
	inline ResidueInteger& operator/=(const ResidueInteger& rhs)
	{
		if (this->mod_ != rhs.mod_)
		{
			throw invalid_argument("The mods of both operands should be equal.");
		}
		this->n_ = getModValue(this->n_ * getModInverse(rhs.n_));
		return *this;
	}
	inline ResidueInteger& operator/=(const Integer& rhs)
	{
		this->n_ = getModValue(this->n_ * getModInverse(getModValue(rhs)));
		return *this;
	}

	inline bool operator==(const ResidueInteger& rhs) const
	{
		return (this->n_ == rhs.n_) && (this->mod_ == rhs.mod_);
	}
	inline bool operator==(const Integer& rhs) const
	{
		return this->n_ == getModValue(rhs);
	}
	friend bool operator==(const Integer& lhs, const ResidueInteger& rhs)
	{
		return rhs.getModValue(lhs) == rhs.n_;
	}

	Integer getN() const { return n_; }

	friend std::ostream& operator<<(std::ostream& lhs, const ResidueInteger& rhs)
	{
		return lhs << rhs.n_;
	}

	friend std::istream& operator>>(std::istream& lhs, ResidueInteger& rhs)
	{
		lhs >> rhs.n_;
		return lhs;
	}
};
using rll = ResidueInteger<ll>;
ll mod = 1000000007ll;
template<> ll rll::global_mod_ = mod;

template<class Integer>
class ResidueCombinatorialCalculator
{
private:
	Integer mod_;
	Integer n_;
	map<Integer, ResidueInteger<Integer>> values_;
public:
	ResidueCombinatorialCalculator(Integer n, Integer mod) : mod_(mod), n_(n)
	{
		values_[0] = ResidueInteger<Integer>(1, mod_);
	}

	ResidueInteger<Integer> calculate(Integer r)
	{
		if (r < 0 || n_ < r)
		{
			return 0;
		}

		if (values_.count(r) >= 1)
		{
			return values_[r];
		}
		if (values_.count(n_ - r) >= 1)
		{
			values_[r] = values_[n_ - r];
			return values_[r];
		}
		if (n_ < r * 2)
		{
			values_[r] = calculate(n_ - r);
			return values_[r];
		}

		values_[r] = calculate(r - 1) * (n_ - r + 1) / r;

		return values_[r];
	}
};
using rccll = ResidueCombinatorialCalculator<ll>;

ll getValue(ll currN, ll currM,
	const vector< ll>& Ss, const vector<ll>& Ts, vector<vector<ll>>& accumRes)
{
	if (accumRes[currN][currM] != -1)
	{
		return accumRes[currN][currM];
	}

	rll currRes = 0;
	currRes += getValue(currN - 1, currM, Ss, Ts, accumRes) +
		getValue(currN, currM - 1, Ss, Ts, accumRes) -
		getValue(currN - 1, currM - 1, Ss, Ts, accumRes);

	if (Ss[currN] == Ts[currM])
	{
		currRes += getValue(currN - 1, currM - 1, Ss, Ts, accumRes) + 1;
	}

	accumRes[currN][currM] = currRes.getN();
	return accumRes[currN][currM];
}

int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll N, M;
	cin >> N >> M;

	vector<vector<ll>> edges(N * 3 + 1, vector<ll>());
	ll u, v;
	for (ll i = 0; i < M; ++i)
	{
		cin >> u >> v;
		edges[u].emplace_back(v + N);
		edges[u + N].emplace_back(v + N * 2);
		edges[u + N * 2].emplace_back(v);
	}

	ll S, T;
	cin >> S >> T;

	queue<pair<ll, ll>> checkQueue;
	vector<bool> haveCheckedNode(N * 3 + 1, false);
	ll resultLength = -3;

	checkQueue.emplace(S, 0);
	haveCheckedNode[S] = true;
	while ( !checkQueue.empty() )
	{
		auto startNodeAndLength = checkQueue.front();
		auto startNode = startNodeAndLength.first;
		auto length = startNodeAndLength.second;
		checkQueue.pop();
		for (const auto& endNode : edges[startNode])
		{
			if (!haveCheckedNode[endNode])
			{
				if (endNode == T)
				{
					resultLength = length + 1;
					goto LOOP_END;
				}
				haveCheckedNode[endNode] = true;
				checkQueue.emplace(endNode, length + 1);
			}
		}
		
	}

LOOP_END:

	cout << resultLength / 3 << endl;


	return 0;
}


Measured memory_bytes: 16768