"""
考察
経路のパターン
1 -> N-1 -> N -> 1
1 -> N -> N-1 -> 1
それ以外は最適にはならないのでは？
"""
from collections import deque


INF = 10**18

N, M = map(int, input().split())
G = [[] for _ in range(N)]
for _ in range(M):
    a, b = map(int, input().split())
    a -= 1; b -= 1
    G[a].append(b)


def bfs(s, t) -> int:
    dist = [INF] * N
    dist[s] = 0
    Q = deque([s])
    while Q:
        now = Q.popleft()
        if now == t:
            return dist[now]
        for nxt in G[now]:
            if dist[nxt] > dist[now] + 1:
                dist[nxt] = dist[now] + 1
                Q.append(nxt)
    return INF


S = 0
T = N-1
StoN_1 = bfs(S, T-1)
StoN = bfs(S, T)
NtoS = bfs(T, S)
N_1toS = bfs(T-1, S)
N_1toN = bfs(T-1, T)
NtoN_1 = bfs(T, T-1)

ans = min(StoN + NtoN_1 + N_1toS, StoN_1 + N_1toN + NtoS)
print(ans if ans < 10**17 else -1)