Михаил Гарриевич Делба , 525 группа DungeonMap 8677			Лёвин Александр, 524 группа DungeonMap 8728
n	1	def build_adj_mat(input_lines):	n	1	def get_adj_matrix(lines):
	2	adj_mat = {}		2	adj_matrix = {}
	3	for line in input_lines:		3	for line in lines:
	4	cave1, cave2 = line.strip().split()		4	cave1, cave2 = line.strip().split()
n	5	adj_mat.setdefault(cave1, set()).add(cave2)	n	5	adj_matrix.setdefault(cave1, set()).add(cave2)
	6	adj_mat.setdefault(cave2, set()).add(cave1)		6	adj_matrix.setdefault(cave2, set()).add(cave1)
	7	return adj_mat		7	return adj_matrix
	8			8
n	9	def can_reach_exit(adj_mat, entrance, exit):	n	9	def can_reach_exit(adj_matrix, ent, exit):
	10	adj_caves = set([entrance])		10	adj_caves = {ent}
	11	fl = True		11	flag = True
	12	while fl:		12	while flag:
	13	fl = False		13	flag = False
	14	for cave1 in list(adj_caves):		14	for cave1 in list(adj_caves):
n	15	for cave2 in adj_mat.get(cave1, []):	n	15	for cave2 in adj_matrix.get(cave1, []):
	16	if cave2 not in adj_caves:		16	if cave2 not in adj_caves:
	17	adj_caves.add(cave2)		17	adj_caves.add(cave2)
n	18	fl = True	n	18	flag = True
	19	return exit in adj_caves		19	return exit in adj_caves
n	20	input_lines = []	n	20	lines = []
	21	while True:		21	while True:
	22	try:		22	try:
	23	line = input().strip()		23	line = input().strip()
	24	if not line:		24	if not line:
	25	break		25	break
n	26	input_lines.append(line)	n	26	lines.append(line)
	27	except EOFError:		27	except EOFError:
	28	break		28	break
t	29	entrance = input_lines[-2]	t	29	ent = lines[-2]
	30	exit = input_lines[-1]		30	exit = lines[-1]
	31	adj_mat = build_adj_mat(input_lines[:-2])		31	adj_mat = get_adj_matrix(lines[:-2])
	32	if can_reach_exit(adj_mat, entrance, exit):		32	if can_reach_exit(adj_mat, ent, exit):
	33	print('YES')		33	print('YES')
	34	else:		34	else:
	35	print('NO')		35	print('NO')

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