n,m=map(int,input().split())
e=[[] for i in range(n)]
for i in range(m):
	a,b=map(int,input().split())
	a-=1
	b-=1
	e[a]+=[b]
X=10**10
v=[X]*n
v[0]=0
q=[0]
for s in q:
	for t in e[s]:
		if v[t]==X:
			v[t]=v[s]+1
			q+=[t]
stot1=v[n-2]
stot2=v[n-1]
v=[X]*n
v[n-2]=0
q=[n-2]
for s in q:
	for t in e[s]:
		if v[t]==X:
			v[t]=v[s]+1
			q+=[t]
t1tos=v[0]
t1tot2=v[n-1]
v=[X]*n
v[n-1]=0
q=[n-1]
for s in q:
	for t in e[s]:
		if v[t]==X:
			v[t]=v[s]+1
			q+=[t]
t2tos=v[0]
t2tot1=v[n-2]
ans=min(stot1+t1tot2+t2tos,stot2+t2tot1+t1tos)
print(ans if ans<X else -1)