# 問題読み間違えていた、頂点1とNを連結したい
# それならダイクストラだろう

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

from heapq import heappush, heappop
INF = 10 ** 18
def dijkstra(s, n, connect): #(始点, ノード数)
    distance = [INF] * n
    que = [(0, s)] #(distance, node)
    distance[s] = 0
    confirmed = [False] * n # ノードが確定済みかどうか
    while que:
        w,v = heappop(que)
        if distance[v]<w:
            continue
        confirmed[v] = True
        for to, cost in connect[v]: # ノード v に隣接しているノードに対して
            if confirmed[to] == False and distance[v] + cost < distance[to]:
                distance[to] = distance[v] + cost
                heappush(que, (distance[to], to))
    return distance

distance = dijkstra(1, N+1, edges)
#print(distance)

ans = distance[N]
if ans == INF:
    print(-1)
else:
    print(ans)