n,m=map(int,input().split())
g=[[] for _ in range(n+1)]
from collections import deque
for _ in range(m):
    a,b=map(int,input().split())
    g[a].append(b)
    g[b].append(a)
d=[-1]*(n+1)
d[1]=0
q=deque()
q.append(1)
while q:
    now=q.popleft()
    for to in g[now]:
        if d[to]==-1:
            d[to]=d[now]+1
            q.append(to)
print(d[n])