import itertools

N = input()

LIST = []

for i in range(N):
  LIST.append(raw_input())


BBB = [i for i in itertools.permutations(LIST)]

for i in range(len(BBB)):
  CCC = BBB[i]
  OK = 1
  DDD = CCC[0]
  for j in range(N-1):
    if CCC[j+1] == DDD:
      OK = 0
      break
    DDD = CCC[j+1]
  if OK == 1:
    print "YES"
    break
if OK == 0:
  print "NO"