import sys
input = lambda: sys.stdin.readline()[:-1]
sys.setrecursionlimit(1000000)

import heapq

n, m = map(int, input().split())
G = [[] for i in range(n)]

for i in range(m):
    u, v = map(int, input().split())
    G[u-1].append(v-1)


def bfs(u, v, v2):
    q = [u]
    dists = [-1]*n
    dists[u] = 0
    now = 1
    while q:
        new_q = []
        for i in q:
            for j in G[i]:
                if dists[j] == -1:
                    new_q.append(j)
                    dists[j] = now
        now += 1
        q = new_q
    return dists[v], dists[v2]

p, pp = bfs(0, n-1, n-2)
ppp, pppp = bfs(n-1, 0, n-2)
ppppp, pppppp = bfs(n-2, 0, n-1)

if p == -1 or pppp == -1 or ppppp == -1:
    if pp == -1 or ppp == -1 or pppppp == -1:
        print(-1)
    else:
        print(pp+ppp+pppppp)
elif pp == -1 or ppp == -1 or pppppp == -1:
    print(p+pppp+ppppp)
else:
    print(min(p+pppp+ppppp, pp+ppp+pppppp))