from collections import deque

n,m = map(int,input().split())
e = [[] for i in range(n)]
for _ in range(m):
    a,b = [int(x)-1 for x in input().split()]
    e[a].append(b)
    e[b].append(a)

dis = [n+1]*n
dis[0] = 0
q = deque([0])
while q:
    now = q.popleft()
    for nex in e[now]:
        if dis[nex] > dis[now]+1:
            q.append(nex)
            dis[nex] = dis[now]+1
if dis[-1] == n+1:
    print(-1)
else:
    print(dis[-1])