N,M = map(int,input().split())
G = [[] for i in range(N)]
for _ in range(M):
    a,b = map(int,input().split())
    G[a - 1].append(b - 1)
    G[b - 1].append(a - 1)
    
from collections import deque
q = deque([])
q.append([0, -1, 0])
visited = [False] * N
while q:
    v,p,c = q.popleft()
    if visited[v]:
        continue
    visited[v] = True
    if v == N - 1:
        print(c)
        break
    for child in G[v]:
        if child == p:
            continue
        if visited[child]:
            continue
        q.append([child, v, c + 1])
else:
    print(-1)