import sys
input = sys.stdin.readline
N,M = map(int,input().split())
UV = [tuple(map(int,input().split())) for _ in range(M)]

es = [[] for _ in range(N)]
for u,v in UV:
    u,v = u-1,v-1
    es[u].append(v)

from collections import deque
INF = 10**18
def bfs(s,g):
    dist = [INF] * N
    dist[s] = 0
    q = deque([s])
    while q:
        v = q.popleft()
        if v==g:
            return dist[v]
        for to in es[v]:
            if dist[to] <= dist[v] + 1: continue
            dist[to] = dist[v] + 1
            q.append(to)
    return INF

ans1 = bfs(0,N-2) + bfs(N-2,N-1) + bfs(N-1,0)
ans2 = bfs(0,N-1) + bfs(N-1,N-2) + bfs(N-2,0)

ans = min(ans1, ans2)
print(-1 if ans >= INF else ans)