from collections import deque

N,M = map(int,input().split())
l = [[] for _ in range(N)]
for _ in range(M):
  a,b = map(int,input().split())
  a -= 1
  b -= 1
  l[a].append(b)
  l[b].append(a)
INF = 10**18
visited = [INF]*N
que = deque([(0,0)])
while que:
  i,c = que.pop()
  for j in l[i]:
    if c+1 < visited[j]:
      que.append((j,c+1))
      visited[j] = c+1

ans = -1
if visited[N-1] != INF:
  ans = visited[N-1]
print(ans)