import sys
#sys.setrecursionlimit(10 ** 6)
INF = float('inf')
#10**20に変えるのもあり
MOD = 10**9 + 7
MOD2 = 998244353
def solve():
    def II(): return int(sys.stdin.readline())
    def LI(): return list(map(int, sys.stdin.readline().split()))
    def LC(): return list(input())
    def IC(): return [int(c) for c in input()]
    def MI(): return map(int, sys.stdin.readline().split())
    N,M = MI()
    Graph = [[] for n in range(N + 1)]
    for i in range(M):
        A, B = MI()
        Graph[A].append((B,1))
        Graph[B].append((A,1))
    from heapq import heappush, heappop
    def dijkstra(s, n):  # (始点, ノード数)
        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:
            v = heappop(hq)[1]  # ノードを pop する
            if seen[v]:
                continue
            seen[v] = True
            for to, cost in Graph[v]:  # ノード v に隣接しているノードに対して
                if seen[to] == False and dist[v] + cost < dist[to]:
                    dist[to] = dist[v] + cost
                    heappush(hq, (dist[to], to))
        return dist
    Dist = dijkstra(1, N+1)
    #print(Dist)
    if(Dist[N] == INF):
        print(-1)
    else:
        print(Dist[N])
    return
solve()