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

PASS = [False]*(N+1)

d = collections.deque()
d.append((1,0))

while len(d):
    p,cnt=d.pop()
    if p==N:
        print(cnt)
        exit()
    for n in L[p]:
        if PASS[n]==False:
            PASS[n]=True
            d.append((n,cnt+1))

print(-1)