import itertools

N = input()

LIST = [raw_input() for i in range(N)]

BBB = [i for i in itertools.permutations(LIST, N)]
for I in BBB:
  BE = I[0]
  OK = 1
  for j in range(N-1):
    if I[j+1] == BE:
      OK = 0
      break
  if OK == 1:
    print "YES"
    break
if OK == 0:
  print "NO"