from collections import defaultdict,deque
N,M=map(int,input().split())
G=[[] for _ in range(4*N+1)]
#Nで割った余りが
#0 ->F,F
#1 -> T,F
#2 -> F,T
#3 -> T,T
for i in range(M):
    u,v=map(int,input().split())
    G[u].append(v)
    G[N+u].append(N+v)
    G[2*N+u].append(2*N+v)
    G[3*N+u].append(3*N+v)
G[N-1].append(N+N-1)
G[2*N+N-1].append(3*N+N-1)
G[N].append(2*N+N)
G[N+N].append(2*N+N+N)
dis=[-1 for _ in range(4*N+1)]
que=deque([1])
seen=[False]*(4*N+1)
seen[1]=True
dis[1]=0
while len(que):
    p=que.popleft()
    for q in G[p]:
        if(not seen[q]):
            seen[q]=True
            dis[q]=dis[p]+1
            que.append(q)
print(max(dis[3*N+1]-2,-1))