### BUGGY CODE ###

import sys
sys.setrecursionlimit(10**7)
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def II(): return int(sys.stdin.readline())
def SI(): return sys.stdin.readline().strip()
INF = 10 ** 18
MOD = 10 ** 9 + 7



import collections
import heapq

class Dijkstra():
    def __init__(self):
        self.e = collections.defaultdict(list)


    def add(self, u, v, d, directed=False):
        """
        0-indexedでなくてもよいことに注意
        u = from, v = to, d = cost
        directed = Trueなら、有向グラフである
        """
        if directed is False:
            self.e[u].append([v, d])
            self.e[v].append([u, d])
        else:
            self.e[u].append([v, d])


    def delete(self, u, v):
        self.e[u] = [_ for _ in self.e[u] if _[0] != v]
        self.e[v] = [_ for _ in self.e[v] if _[0] != u]


    def Dijkstra_search(self, s):
        """
        0-indexedでなくてもよいことに注意
        s = 始点
        return: 始点から各点までの最短経路
        """
        d = collections.defaultdict(lambda: float('inf'))
        d[s] = 0
        q = []
        heapq.heappush(q, (0, s))
        v = collections.defaultdict(bool)
        while len(q):
            k, u = heapq.heappop(q)
            if v[u]:  # DefaulfではFalseとなる
                continue


            for uv, ud in self.e[u]:
                if v[uv]:
                    continue
                vd = k + ud
                if d[uv] > vd:
                    d[uv] = vd
                    heapq.heappush(q, (vd, uv))


        return d



def main(): 
    # ダイクストラ法を用いて回答する。
    N, M = LI()
    graph = []
    from collections import defaultdict
    reversed_graph_dict = defaultdict(list)
    for _ in range(M):
        u, v = LI()
        graph.append([u,v])
        reversed_graph_dict[v].append(u)
    S, T = LI()

    dij = Dijkstra()

    # 三段先のグラフを先に作っておく。
    three_steps_dict = defaultdict(list)
    for i in range(1, N+1):
        added = set()
        first_step = reversed_graph_dict[i]
        second_step = []
        for x in first_step:
            second_step += reversed_graph_dict[x]
        third_step = []
        for x in set(second_step):
            for y in set(reversed_graph_dict[x]):
                if (i,y) not in added:
                    dij.add(i,y,1,directed=True)
                    added.add((i,y))

    ans = dij.Dijkstra_search(s=T)[S]
    print(ans if ans != float('inf') else -1)



main()

### FIXED CODE ###

import sys
sys.setrecursionlimit(10**7)
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def II(): return int(sys.stdin.readline())
def SI(): return sys.stdin.readline().strip()
INF = 10 ** 18
MOD = 10 ** 9 + 7

def main(): 
    N, M = LI()
    graph = []
    from collections import defaultdict
    reversed_graph_dict = defaultdict(list)
    for _ in range(M):
        u, v = LI()
        graph.append([u,v])
        reversed_graph_dict[v].append(u)
    S, T = LI()

    # print(reversed_graph_dict)

    import collections
    def bfs(graph, root):
        seen, queue = set([root]), collections.deque([[root, 0],])
        first_step_seen = set([])
        second_step_seen = set([])
        while queue:
            vertex, tries = queue.popleft()
            tries += 1

            # 三段先を見に行く。
            first_step = graph[vertex]

            second_step = []
            for x in set(first_step) - first_step_seen:
                second_step += graph[x]
            first_step_seen |= set(first_step)

            third_step = []
            for x in set(second_step) - second_step_seen:
                third_step += graph[x]
            second_step_seen |= set(second_step)

            for node in set(third_step):
                if node == S:
                    print(tries)
                    return
                if node not in seen:
                    seen.add(node)
                    queue.append([node, tries])
        print(-1)
        return

    bfs(reversed_graph_dict, T)


main()