from collections import deque

n, m = map(int, input().split())

g = [[] for _ 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)

src = 0
dst = n - 1
dist = [None for _ in range(n)]
dist[src] = 0
dq = deque()
dq.appendleft(src)
while len(dq) > 0:
  cur = dq.pop()
  for nxt in g[cur]:
    if not dist[nxt] is None:
      continue
    dist[nxt] = dist[cur] + 1
    dq.appendleft(nxt)

if dist[dst] is None:
  print("-1")
else:
  print(dist[dst])