from collections import deque
def bfs(s):
    dq=deque()
    dq.append([s,0])
    high=[10**9 for _ in range(N)]
    high[s]=0
    while len(dq)!=0:
        p,h=dq.popleft()
        for e in edge[p]:
            if  high[e]==10**9:
                dq.append([e,h+1])
                high[e]=h+1
    return high

N,M=map(int,input().split())
edge=[[] for _ in range(N)]
for i in range(M):
    u,v=map(int,input().split())
    u-=1
    v-=1
    edge[u].append(v)
dist0=bfs(0)
distn_=bfs(N-2)
distn=bfs(N-1)
ans=min(dist0[N-2]+distn_[N-1]+distn[0],dist0[N-1]+distn[N-2]+distn_[0])
if ans>10**9:
    print(-1)
else:
    print(ans)