from collections import deque
n,m=map(int,input().split())
path=[[] for _ in range(n)]
for i in range(m):
    a,b=map(int,input().split())
    path[a-1].append(b-1)
    path[b-1].append(a-1)
ans=[10**9]*n
ans[0]=0
queue=deque()
queue.append(0)
while len(queue)!=0:
    num=queue.popleft()
    for i in path[num]:
        if ans[i]>ans[num]+1:
            ans[i]=ans[num]+1
            queue.append(i)
if ans[-1]==10**9:
    print(-1)
else:
    print(ans[-1])