def INT():
    return int(input())


def MI():
    return map(int, input().split())


def LI():
    return list(map(int, input().split()))


import heapq


def dijkstra(n, G, start=0):
    INF = 10**18
    dist = [INF] * n
    dist[start] = 0
    pq = []
    heapq.heappush(pq, (0, start))
    visited = [False] * n

    while pq:
        now = heapq.heappop(pq)[1]
        visited[now] = True
        for nxt, cost in G[now]:
            if dist[nxt] > dist[now] + cost and not visited[nxt]:
                dist[nxt] = dist[now] + cost
                heapq.heappush(pq, (dist[now] + cost, nxt))

    return dist


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

dist1 = dijkstra(N, G, 0)  # from 0
dist2 = dijkstra(N, G, N - 2)  # from N - 2
dist3 = dijkstra(N, G, N - 1)  # from N - 1
INF = 10**18
if (dist1[N - 2] == INF or dist2[N - 1] == INF or dist3[0] == INF) or (
    dist1[N - 1] == INF or dist3[N - 2] == INF or dist2[0] == INF
):
    print(-1)
    exit()

cand1 = dist1[N - 2] + dist2[N - 1] + dist3[0]
cand2 = dist1[N - 1] + dist3[N - 2] + dist2[0]
ans = min(cand1, cand2)
print(ans)