from collections import deque

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

visited = [[-1]*(1<<2) for _ in range(N)]
visited[0][0] = 0
que = deque()
que.append((0, 0))
while que:
    n, bit = que.popleft()
    for v in G[n]:
        nbit = bit
        if v == N-2:
            nbit |= 1<<0
        if v == N-1:
            nbit |= 1<<1
        if visited[v][nbit] == -1:
            visited[v][nbit] = visited[n][bit]+1
            que.append((v, nbit))

print(visited[0][-1])