# import pypyjit;pypyjit.set_param("max_unroll_recursion=-1")
import os
import sys

# from bisect import *
from collections import *

# from datetime import *
# from decimal import *  # PyPyだと遅い
# from heapq import *
from itertools import *

# from math import gcd, lcm
# from random import *
# from string import *

# import numpy as np
# from atcoder.dsu import *
# from atcoder.segtree import *
# from more_itertools import *
# from sortedcontainers import *

# sys.setrecursionlimit(10**7)  # PyPyは呪文を付ける
# sys.set_int_max_str_digits(0)
INF = 1 << 61
MOD = 998244353
# MOD = 10**9 + 7
IS_ATCODER = os.getenv("ATCODER", 0)

File = sys.stdin


def input():
    return File.readline().replace("\n", "")


# ///////////////////////////////////////////////////////////////////////////
def bfs(G, start):
    """
    グラフで幅優先探索をする関数
    seenとdistを配列で管理
    何度も繰り返す場合はseenの生成がO(N)なので
    辞書で管理するものを使う

    引数:
    G (defaultdict): グラフ
    start (int): 始点

    戻り値:
    dist (list): 始点からの距離
    """
    que = deque()
    seen = [0] * N
    dist = [INF] * N

    que.append(start)
    seen[start] = True
    dist[start] = 0
    while que:
        for _ in range(len(que)):
            pop = que.popleft()
            for i in G[pop]:
                if seen[i] == False:
                    seen[i] = True
                    dist[i] = dist[pop] + 1
                    que.append(i)

    return dist


N, M = map(int, input().split())
edges = [list(map(int, input().split())) for _ in range(M)]

G = [set() for _ in range(N)]
for i, j in edges:
    G[i - 1].add(j - 1)

dist1 = bfs(G, 0)
dist2 = bfs(G, N - 1)
dist3 = bfs(G, N - 2)
ans = INF

ans = min(ans, dist1[N - 1] + dist2[N - 2] + dist3[0])
ans = min(ans, dist1[N - 2] + dist3[N - 1] + dist2[0])

print(ans if ans != INF else -1)