import heapq

N,M = map(int,input().split())
a = [0] * M
b = [0] * M
for i in range(M):
    a[i],b[i] = map(int,input().split())
v = [[] for _ in range(N+1)]
for i in range(M):
    v[a[i]].append(b[i])
    v[b[i]].append(a[i])
    
INF = 10 ** 6
dis = [INF] * (N+1)
dis[1] = 0
q = []
heapq.heapify(q)
heapq.heappush(q,(dis[1],1))
while len(q):
    d,s = heapq.heappop(q)
    if dis[s] < d:continue

    for i in v[s]:
        if dis[i] > dis[s] + 1:
            dis[i] = dis[s] + 1
            heapq.heappush(q,(dis[i],i))
if dis[N] == INF:print(-1)
else:print(dis[N])