import sys
from collections import deque
input = sys.stdin.buffer.readline

def solve(N, G):
    INF = 10 ** 9
    # bfs
    d = deque([0])
    dist = [INF] * N
    dist[0] = 0
    while d:
        v = d.popleft()
        if v == N - 1:
            return dist[v]
        dx = dist[v] + 1
        for x in G[v]:
            if dist[x] == INF:
                d.append(x)
                dist[x] = dx
    return -1


N, M = map(int, input().split())
G = [[] for _ in range(N)]
for _ in range(M):
    a, b = tuple(int(x) - 1 for x in input().split())
    G[a].append(b)
    G[b].append(a)

print(solve(N, G))