import sys
input = sys.stdin.readline

from collections import deque
inf = 10 ** 18
def bfs(G, s, N):
    Q = deque([])
    dist = [inf] * N
    par = [inf] * N
    dist[s] = 0
    for u in G[s]:
        par[u] = s
        dist[u] = 1
        Q.append(u)

    while Q:
        u = Q.popleft()
        for v in G[u]:
            if dist[v] != inf:
                continue
            dist[v] = dist[u] + 1
            par[v] = u
            Q.append(v)
            
    return dist


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

D0 = bfs(G, 0, N)
D1 = bfs(G, N-2, N)
D2 = bfs(G, N-1, N)
ans = min(D0[N - 2] + D1[N - 1] + D2[0], D0[N - 1] + D2[N - 2] + D1[0])
print(ans) if ans < 10 ** 10 else print(-1)