from collections import deque
N,M = map(int,input().split())
E = [[] for i in range(N)]
for _ in range(M):
    a,b = map(int,input().split())
    E[a-1].append(b-1)
    E[b-1].append(a-1)
q = deque()
q.append(0)
dp = [0]*N
while q:
    x = q.pop()
    for y in E[x]:
        if y != 0 and dp[y] == 0:
            dp[y] = dp[x]+1
            q.appendleft(y)
if dp[-1] == 0:
    ans = -1
else:
    ans = dp[-1]
print(ans)