import collections
import heapq


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

    def add(self, u, v, d):
        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 search(self, s):
        """
        :param s: 始点
        :return: 始点から各点までの最短経路
        """
        d = collections.defaultdict(lambda: 10**8)
        d[s] = 0
        q = []
        heapq.heappush(q, (0, s))
        v = collections.defaultdict(bool)
        while len(q):
            k, u = heapq.heappop(q)
            if v[u]:
                continue
            v[u] = True

            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

N,M = map(int, input().split())
uv = [list(map(int, input().split())) for _ in range(M)]
G = Dijkstra()
RG = Dijkstra()
for u,v in uv:
    G.add(u,v,1)
D1 = G.search(1)
DN = G.search(N)
DN1 = G.search(N-1)
ans = min(D1[N]+DN[N-1]+DN1[1],D1[N-1]+DN1[N]+DN[1])
if ans<10**8:
    print(ans)
else:
    print(-1)