INF = 1e18
from collections import deque
N, M = map(int, input().split())
G = [list() for _ in range(N)]
for i in range(M):
    u, v = map(int, input().split())
    u -= 1
    v -= 1
    G[u].append(v)

dist = [INF] * N
dist[0] = 0
que = deque([0])
while que:
    pos = que.popleft()
    for nex in G[pos]:
        if dist[nex] == INF:
            dist[nex] = dist[pos] + 1
            que.append(nex)

dist_from_N = [INF] * N
dist_from_N[-1] = 0
que = deque([N - 1])
while que:
    pos = que.popleft()
    for nex in G[pos]:
        if dist_from_N[nex] == INF:
            dist_from_N[nex] = dist_from_N[pos] + 1
            que.append(nex)

dist_from_N_1 = [INF] * N
dist_from_N_1[-2] = 0
que = deque([N - 2])
while que:
    pos = que.popleft()
    for nex in G[pos]:
        if dist_from_N_1[nex] == INF:
            dist_from_N_1[nex] = dist_from_N_1[pos] + 1
            que.append(nex)

ans1 = dist[-2] + dist_from_N_1[-1] + dist_from_N[0]
ans2 = dist[-1] + dist_from_N[-2] + dist_from_N_1[0]
ans = min(ans1, ans2)
if ans >= INF:
    ans = -1
print(ans)