from collections import deque

def bfs(s):
    dist = [INF] * N
    dist[s] = 0
    que = deque([s])
    while que:
        v = que.popleft()
        for nv in g[v]:
            if dist[nv] == INF:
                dist[nv] = dist[v] + 1
                que.append(nv)
    return dist

INF = 1 << 60
N, M = map(int, input().split())
g = [[] for _ in range(N)]
for _ in range(M):
    u, v = map(int, input().split())
    u, v = u - 1, v - 1
    g[u].append(v)
dist_1 = bfs(0)
dist_N = bfs(N - 1)
dist_Nm1 = bfs(N - 2)
ans = min(dist_1[-1] + dist_N[N - 2] + dist_Nm1[0], dist_1[N - 2] + dist_Nm1[-1] + dist_N[0])
print(ans if ans < INF else -1)