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

INF = 10**18
def f(start):
    dist = [INF for _ in range(N)]
    
    q = deque([(0,start)])
    while(q):
        v,now = q.popleft()

        if(dist[now] <= v):continue
        dist[now] = v

        for i in edge[now]:
            q.append((v+1,i))
    return dist

from_0 = f(0)
from_l = f(N-1)
from_ll = f(N-2)

ans = min(from_0[N-1] + from_l[N-2] + from_ll[0], from_0[N-2] + from_ll[N-1] + from_l[0])
print(ans if ans < INF else -1)