import sys,math
#if len(sys.argv)==2:sys.stdin=open(sys.argv[1])
from collections import defaultdict,deque
from itertools import combinations,permutations,accumulate,product
from bisect import bisect,bisect_left,bisect_right
from heapq import heappop,heappush,heapify
#from sortedcontainers import SortedList,SortedSet
def input(): return sys.stdin.readline().rstrip()
def ii(): return int(input())
def ms(): return map(int, input().split())
def li(): return list(map(int,input().split()))
N,M = ms()
G = [set() for _ in range(N+1)]
for _ in range(M):
  u,v = ms()
  G[u].add(v)
  
ans = 10**18
tmp = 0
ng = False
visited = [False]*(N+1)
visited[1] = True
dist = [-1]*(N+1)
dist[1] = 0
que = deque()
que.append(1)
end = False
while que:
  v = que.popleft()
  visited[v] = True
  for v2 in G[v]:
    if visited[v2]: continue
    dist[v2] = dist[v] + 1
    if v2==N-1:
      end = True
      break
    que.append(v2)
  if end: break

if end==False: ng = True
if ng==False:
  tmp += dist[N-1]
  visited = [False]*(N+1)
  visited[N-1] = True
  dist = [-1]*(N+1)
  dist[N-1] = 0
  que = deque()
  que.append(N-1)
  end = False
  while que:
    v = que.popleft()
    visited[v] = True
    for v2 in G[v]:
      if visited[v2]: continue
      dist[v2] = dist[v] + 1
      if v2==N:
        end = True
        break
      que.append(v2)
    if end: break
  if end==False: ng = True
    
  tmp += dist[N]
  end = False
  visited = [False]*(N+1)
  visited[N] = True
  dist = [-1]*(N+1)
  dist[N] = 0
  que = deque()
  que.append(N)
  end = False
  while que:
    v = que.popleft()
    visited[v] = True
    for v2 in G[v]:
      if visited[v2]: continue
      dist[v2] = dist[v] + 1
      if v2==1:
        end = True
        break
      que.append(v2)
    if end: break
  if end==False: ng = True
  
  tmp += dist[1]
  if ng==False: ans = min(ans,tmp)
  
tmp = 0
ng = False
visited = [False]*(N+1)
visited[1] = True
dist = [-1]*(N+1)
dist[1] = 0
que = deque()
que.append(1)
end = False
while que:
  v = que.popleft()
  visited[v] = True
  for v2 in G[v]:
    if visited[v2]: continue
    dist[v2] = dist[v] + 1
    if v2==N:
      end = True
      break
    que.append(v2)
  if end: break

if end==False: ng = True
if ng==False:
  tmp += dist[N]
  visited = [False]*(N+1)
  visited[N] = True
  dist = [-1]*(N+1)
  dist[N] = 0
  que = deque()
  que.append(N)
  end = False
  while que:
    v = que.popleft()
    visited[v] = True
    for v2 in G[v]:
      if visited[v2]: continue
      dist[v2] = dist[v] + 1
      if v2==N-1:
        end = True
        break
      que.append(v2)
    if end: break
  if end==False: ng = True
    
  tmp += dist[N-1]
  end = False
  visited = [False]*(N+1)
  visited[N-1] = True
  dist = [-1]*(N+1)
  dist[N-1] = 0
  que = deque()
  que.append(N-1)
  end = False
  while que:
    v = que.popleft()
    visited[v] = True
    for v2 in G[v]:
      if visited[v2]: continue
      dist[v2] = dist[v] + 1
      if v2==1:
        end = True
        break
      que.append(v2)
    if end: break
  if end==False: ng = True
  
  tmp += dist[1]
  if ng==False: ans = min(ans,tmp)

if ans!=10**18: print(ans)
else: print(-1)