不易

bùyì
  1. not easy to do sth
  2. difficult
  3. unchanging

CEDICT

Entry Composition

1 不 bù
  1. no; not so
  2. (bound form) not; un-
2 易 yì
  1. easy
  2. amiable
  3. to change
  4. to exchange
  5. prefix corresponding to the English adjective suffix "-able" or "-ible"