from collections import deque
import sys
input=sys.stdin.readline

N,M=map(int,input().split())
E=[[] for _ in range(N+1)]
for _ in range(M):
    a,b=map(int,input().split())
    E[a].append(b)
    E[b].append(a)

X=[-1]*(N+1); X[1]=0
Q=deque([1])
while Q:
    x=Q.popleft()

    for y in E[x]:
        if X[y]==-1:
            X[y]=X[x]+1
            Q.append(y)

print(X[N])