class Input_kyopro:
    def II(self): return int(input())
    def MI(self): return map(int,input().split())
    def MS(self): return map(str,input().split())
    def LMI(self): return list(self.MI())
    def LMS(self): return list(self.MS())
    def LLI(self,N): return [self.LMI() for _ in range(N)]
    def LLS(self,N): return [self.LMS() for _ in range(N)]
    def LS(self,N): return [input() for _ in range(N)]
    def LSL(self,N): return [list(input()) for _ in range(N)]
    def LI(self,N): return [self.II() for _ in range(N)]
I=Input_kyopro()
#入力
from collections import deque
N,M=I.MI()
uv=I.LLI(M)
G=[[] for _ in range(N)]
for u,v in uv:
    u-=1
    v-=1
    G[u].append(v)
q=deque()
q.append(0)
dist=[-1]*N
dist[0]=0
while q:
    pos=q.popleft()
    for nex in G[pos]:
        if dist[nex]!=-1:
            continue
        dist[nex]=dist[pos]+1
        q.append(nex)
q=deque()
q.append(N-1)
dist1=[-1]*N
dist1[N-1]=0
while q:
    pos=q.popleft()
    for nex in G[pos]:
        if dist1[nex]!=-1:
            continue
        dist1[nex]=dist1[pos]+1
        q.append(nex)
q=deque()
q.append(N-2)
dist2=[-1]*N
dist2[N-2]=0
while q:
    pos=q.popleft()
    for nex in G[pos]:
        if dist2[nex]!=-1:
            continue
        dist2[nex]=dist2[pos]+1
        q.append(nex)
ans=float('inf')
if dist[-1]==-1 or dist[-2]==-1:
    print(-1)
else:
    if dist1[N-2]!=-1 and dist2[0]!=-1:
        ans=min(ans,dist[N-1]+dist1[N-2]+dist2[0])
    if dist2[N-1]!=-1 and dist1[0]!=-1:
        ans=min(ans,dist[N-2]+dist2[N-1]+dist1[0])
    if ans==float('inf'):
        print(-1)
    else:
        print(ans)