from collections import deque

N,M=map(int,input().split())
E=[[] for i in range(N+1)]

DIS=[1<<30]*(N+1)
DIS[1]=0
Q=deque([1])

for i in range(M):
    x,y=map(int,input().split())
    E[x].append(y)
    E[y].append(x)

while Q:
    x=Q.popleft()
    for to in E[x]:
        if DIS[to]>DIS[x]+1:
            DIS[to]=DIS[x]+1
            Q.append(to)

if DIS[N]>N+10:
    print(-1)
else:
    print(DIS[N])