import sys
sys.setrecursionlimit(10**5)
input=sys.stdin.readline
import pypyjit
pypyjit.set_param('max_unroll_recursion=-1')

from collections import deque
def bfs(v):
    que=deque();que.append(v)
    dist=[-1]*n;dist[v]=0
    while que:
        v=que.popleft()
        for nv in graph[v]:
            if dist[nv]!=-1:continue    
            dist[nv]=dist[v]+1
            que.append(nv)
    
    dist=list(map(lambda x:x if x!=-1 else float('INF'),dist))
    return dist

n,m=map(int,input().split())
graph=[[] for _ in range(n)]
for _ in range(m):
    u,v=map(lambda x:int(x)-1,input().split())
    graph[u].append(v)

dist1=bfs(0)
distn_1=bfs(n-2)
distn=bfs(n-1)

ans=float('INF')
ans=min(ans,dist1[n-2]+distn_1[n-1]+distn[0])
ans=min(ans,dist1[n-1]+distn[n-2]+distn_1[0])

print(ans if ans!=float('INF') else -1)