from collections import deque, defaultdict

n, m = map(int, input().split())
edge = defaultdict(list)
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    edge[a].append(b)
    edge[b].append(a)

ans = -1
visited = set([0])
Que = deque([(0, 0)])
while Que:
    curr, cnt = Que.popleft()
    if curr == n - 1:
        ans = cnt
        break
    for np in edge[curr]:
        if np in visited:
            continue
        visited.add(np)
        Que.append((np, cnt + 1))
print(ans)