from collections import deque

INF = 10 ** 17

N, M = map(int, input().split())
graph = [[] for _ in range(N)]
for i in range(M):
    u, v = map(int, input().split())
    graph[u - 1].append(v - 1)


def bfs(v):
    dq = deque()
    isvisited = [False for _ in range(N)]
    dist = [INF for _ in range(N)]
    isvisited[v] = True
    dist[v] = 0
    dq.append(v)
    while len(dq):
        p = dq.popleft()
        for x in graph[p]:
            if isvisited[x]:
                continue
            isvisited[x] = True
            dq.append(x)
            dist[x] = dist[p] + 1
    return dist


dist1 = bfs(0)
distn1 = bfs(N - 2)
distn = bfs(N - 1)

ans = min(dist1[N - 2] + distn1[N - 1] + distn[0],
          dist1[N - 1] + distn[N - 2] + distn1[0])
if ans >= INF:
    print(-1)
else:
    print(ans)