n, m = map(int, input().split())
G = [[] for i in range(n)]
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1; b -= 1
    G[a].append(b)
    G[b].append(a)
from collections import deque
Q = deque([0])
dist = [-1] * n
dist[0] = 0
while Q:
    now = Q.popleft()
    for nex in G[now]:
        if dist[nex] == -1:
            dist[nex] = dist[now] + 1
            Q.append(nex)
print(dist[-1])