from heapq import heappop, heappush
INF = float('inf')

def dijkstra(s, n):
    dist = [INF] * n
    hq = [(0, s)] # (distance, node)
    dist[s] = 0
    seen = [False] * n # ノードが確定済みかどうか
    while hq:
        dis, v = heappop(hq)
        if dist[v] < dis:
            continue
        seen[v] = True
        for to, cost in G[v]:
            if seen[to] == False and dist[v] + cost < dist[to]:
                dist[to] = dist[v] + cost
                heappush(hq, (dist[to], to))
    return dist

N, M = map(int, input().split())
G = [list() for _ in range(N)]
for i in range(M):
    a, b = map(int, input().split())
    a-=1;b-=1
    G[a].append((b,1))
    G[b].append((a,1))

d = dijkstra(0, N)
print(-1) if d[-1]==INF else print(d[-1])