N,M = map(int, input().split())
L = [[] for i 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)

def dfs(p,cnt):
    if p==N:
        print(cnt)
        exit()
    if PASS[p]==False:
        PASS[p]=True
        for n in L[p]:
            dfs(n,cnt+1)

dfs(1,0)
print(-1)