import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static string[] s_rd;
T RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
string RDR()() { return readln.chomp; }
T[] ARR(T = long)(in string str, T fix = 0) { auto r = str.split.to!(T[]); r[] += fix; return r; }
size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
long lcm(long x, long y) { return x * y / gcd(x, y); }

long mod = 10^^9 + 7;
//long mod = 998244353;
//long mod = 1_000_003;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }

void main()
{
	auto N = RD;
	auto M = RD;
	auto edges = new long[][](N);
	//auto edges2 = new long[][](N);
	foreach (i; 0..M)
	{
		auto u = RD-1;
		auto v = RD-1;
		edges[u]  ~= v;
		//edges2[v] ~= u;
	}
	auto S = RD-1;
	auto T = RD-1;

	/*auto d = new bool[3][](N);
	{
		d[S][0] = true;
		long[2][] open = [[S, 0]];
		while (!open.empty)
		{
			auto n    = open.back; open.popBack;
			auto from = n[0];
			auto c    = n[1];
			foreach (to; edges[from])
			{
				auto nc = (c+1) % 3;
				if (d[to][nc]) continue;
				d[to][nc] = true;
				open ~= [to, nc];
			}
		}
	}*/

	long ans;
	auto cost = new long[][](N, 3);
	foreach (ref e; cost)
		e.fill(long.max);
	cost[S][0] = 0;
	long[2][] open = [[S, 0]];
	while (!open.empty)
	{
		auto n    = open.back; open.popBack;
		auto from = n[0];
		auto c    = n[1];
		foreach (to; edges[from])
		{
			auto nc  = c + 1;
			auto cnt = nc % 3;
			//if (d[to][pc] == false) continue;
			if (nc < cost[to][cnt])
			{
				cost[to][cnt] = nc;
				open         ~= [to, nc];
			}
		}
	}
	debug writeln(cost);
	
	if (cost[T][0] == long.max)
	{
		writeln(-1);
	}
	else
	{
		writeln(cost[T][0] / 3);
	}

	stdout.flush();
	debug readln();
}