import heapq

n, m = map(int, input().split())
edges = [[] for _ in range(n)]
for _ in range(m):
    u, v = map(int, input().split())
    u, v = u - 1, v - 1
    edges[u].append(v)

INF = float("inf")
def dijkstra(s, g):
    dist = [INF] * n
    que = [(0, s)]
    heapq.heapify(que)
    while que:
        cost, crt = heapq.heappop(que)
        if dist[crt] <= cost:
            continue
        dist[crt] = cost
        for nxt in edges[crt]:
            if dist[nxt] <= cost + 1:
                continue
            heapq.heappush(que, (cost + 1, nxt))
    return dist[g]

ans = INF
for l in ((0, n - 1, n - 2, 0), (0, n - 2, n - 1, 0)):
    tmp = 0
    for a, b in zip(l, l[1:]):
        tmp += dijkstra(a, b)
    ans = min(ans, tmp)
print(-1 if ans == INF else ans)