from collections import deque

N,M=list(map(int, input().split()))
G=[[] for _ in range(N)]
for _ in range(M):
    u,v=list(map(int, input().split()))
    G[u-1].append(v-1)

INF=1<<60

def bfs(s, g):
    d=[INF for _ in range(N)]
    Q=deque()
    d[s]=0
    Q.append(s)
    while Q:
        pos=Q.popleft()
        for nex in G[pos]:
            if d[pos]+1<d[nex]:
                d[nex]=d[pos]+1
                Q.append(nex)
    return d[g]


root1=bfs(0,N-2)+bfs(N-2,N-1)+bfs(N-1,0)
root2=bfs(0,N-1)+bfs(N-1,N-2)+bfs(N-2,0)
ans=min(root1,root2)
if ans>=INF:
    print(-1)
else:
    print(ans)