from collections import deque
N,M = map(int,input().split())
G = {i:[] for i in range(1,N+1)}
for _ in range(M):
    a,b = map(int,input().split())
    G[a].append(b)
    G[b].append(a)
que = deque([1])
dist = [-1]*(N+1)
dist[1] = 0
while que:
    x = que.popleft()
    for y in G[x]:
        if dist[y]<0:
            dist[y] = dist[x]+1
            que.append(y)
print(dist[N])