from queue import Queue
class graph(object):
    def __init__(self, n=None, N=200010):
        self.h = [-1] * N
        self.e = [-1] * N
        self.ne = [-1] * N
        self.n = n
        self.idx = 0
        self.st = [False] *N

    def add(self, a, b):
        idx = self.idx
        self.e[idx] = b
        self.ne[idx] = self.h[a]
        self.h[a] = idx
        self.idx += 1

    def bfs(self, u, t):
        q = Queue()
        q.put([u,0])
        while not q.empty():
            p, lvl = q.get()
            self.st[p] = True
            i = self.h[p]
            while i != -1:
                v = self.e[i]
                if v == t:
                    return lvl + 1
                if not self.st[v]:
                    q.put([v, lvl+1])
                i = self.ne[i]
        return  -1

n, m = map(int, input().split(' '))
g = graph()
for _ in range(m):
    a, b = map(int, input().split(' '))
    g.add(a, b)
print(g.bfs(1, n))