Churras

from collections import *

def bfs(no):
    q = []
    q.append(no)
    vis[no] = True
    dist[no] = 0
    while not len(q) == 0:
        top = q.pop()
        for vizinho in grafo[top]:
            if not vis[vizinho]: q.append(vizinho)
            vis[vizinho] = True
            dist[vizinho] = min(dist[vizinho],dist[top] + 1)


cases, num = map(int, raw_input().split())
grafo = defaultdict(list)
pos = []

for i in range(cases):
    nome, amigo = raw_input().split()
    pos.append(nome)
    pos.append(amigo)

    grafo[nome] += [amigo]
    grafo[amigo] += [nome]

vis = {i:False for i in pos}
dist = {i:100000 for i in pos}

pos = set(pos)
bfs("Rerisson")

new = ""
cont = 0
for i in sorted(dist):
    if 1 <= dist[i] <= num:
        cont += 1
        new += i + "\n"

print cont
print new,