N,M = map(int,input().split())
from collections import defaultdict
from collections import deque
graph = defaultdict(list)
for i in range(M):
    u,v = map(int,input().split())
    graph[u-1].append(v-1)
INF = 10**18

######print(graph)
def bfs(start,goal):
    dist = [INF]*(N)
    dist[start] = 0
    quene = deque()
    quene.append(start)
    while quene:
        v=quene.popleft()

        for i in graph[v]:
            if dist[i] >= dist[v]+1:
                    dist[i] = dist[v]+1
                    quene.append(i)
                    continue

    return dist[goal]

a = bfs(0,N-1)
b = bfs(0,N-2)
c = bfs(N-1,0)
d = bfs(N-1,N-2)
e = bfs(N-2,0)
f = bfs(N-2,N-1)

if INF <= (a+d+e) and INF <= (b+c+f):
    print(-1)
    exit()


print(min((a+d+e),(b+c+f)))