#!/usr/bin/env python3
import sys
from bisect import bisect_left, bisect_right, insort_left, insort_right  # type: ignore
from collections import Counter, defaultdict, deque  # type: ignore
from math import gcd, sqrt, ceil, factorial  # type: ignore
from heapq import heapify, heappop, heappush, heappushpop, heapreplace, merge  # type: ignore
from itertools import accumulate, combinations, permutations, product  # type: ignore
from string import ascii_lowercase, ascii_uppercase # type: ignore

def LI(): return list(map(int, sys.stdin.buffer.readline().split()))
def I(): return int(sys.stdin.buffer.readline())
def LS(): return sys.stdin.buffer.readline().rstrip().decode("utf-8").split()
def S(): return sys.stdin.buffer.readline().rstrip().decode("utf-8")
def IR(n): return [I() for _ in range(n)]
def LIR(n): return [LI() for _ in range(n)]
def SR(n): return [S() for _ in range(n)]
def LSR(n): return [LS() for _ in range(n)]
def SRL(n): return [list(S()) for _ in range(n)]

N,M = LI()
G = [[] for _ in range(N)]
for _ in range(M):
    u,v = LI()
    u-=1
    v-=1
    G[u].append(v)

INF = 10**9
def bfs(s):
    dist = [INF]*N
    dist[s] = 0
    q = deque([s])
    while q:
        v = q.popleft()
        cur = dist[v]
        for nex in G[v]:
            if dist[nex]<INF: continue
            dist[nex] = cur+1
            q.append(nex)
    return dist

d0 = bfs(0)
dn2 = bfs(N-2)
dn1 = bfs(N-1)

ans = min(
    INF,
    d0[N-2]+dn2[N-1]+dn1[0],
    d0[N-1]+dn1[N-2]+dn2[0]
)
print(ans if ans<INF else -1)