N,M=map(int,input().split())
G=[[] for i in range(N)]
for i in range(M):
  a,b=map(int,input().split())
  G[a-1].append(b-1)
from collections import deque
S=deque()
dist=[10**10]*N
dist[0]=0
S.append(0)
while S:
  x=S.popleft()
  for y in G[x]:
    if dist[y]<10**8:
      continue
    dist[y]=dist[x]+1
    S.append(y)
L1=[10**10]*N
L1[N-1]=0
S=deque()
S.append(N-1)
while S:
  x=S.popleft()
  for y in G[x]:
    if L1[y]<10**8:
      continue
    L1[y]=L1[x]+1
    S.append(y)
L2=[10**10]*N
L2[N-2]=0
S=deque()
S.append(N-2)
while S:
  x=S.popleft()
  for y in G[x]:
    if L2[y]<10**8:
      continue
    L2[y]=L2[x]+1
    S.append(y)
result=10**10
ans=dist[N-1]+L1[N-2]+L2[0]
result=min(result,ans)
ans=dist[N-2]+L2[N-1]+L1[0]
result=min(result,ans)
if result>10**8:
  print(-1)
  exit()
print(result)