from collections import deque
INF=pow(2,61)-1
N,M=map(int,input().split())
gr=[[set() for i in range(4)] for i in range(N)]
for i in range(M):
    u,v=map(int,input().split())
    for j in range(4):
        if v==N-1:
            gr[u-1][j].add((v-1,j|1))
        elif v==N:
            gr[u-1][j].add((v-1,j|2))
        else:
            gr[u-1][j].add((v-1,j))
pos=deque([(0,0)])
ans=[[INF]*4 for i in range(N)]
ans[0][0]=0
while len(pos)>0:
    a,b=pos.popleft()
    for i,j in gr[a][b]:
        if ans[i][j]>ans[a][b]+1:
            ans[i][j]=ans[a][b]+1
            pos.append((i,j))
if ans[0][3]>=INF:
    print(-1)
else:
    print(ans[0][3])