program main
  implicit none
  integer::N
  character*3::S
  read *,N
  if(N.ge.4) then
     print '(a)',"YES"
  else if(N.eq.1) then
     print '(a)',"NO"
  else if(N.eq.2) then
     read *,S
     if(S(1:1).eq.S(2:2)) then
        print '(a)',"YES"
     else
        print '(a)',"NO"
     end if
  else if(N.eq.3) then
     read *,S
     if(S(1:1).eq.S(3:3).and.S(1:1).ne.S(2:2)) then
        print '(a)',"NO"
     else
        print '(a)',"YES"
     end if
  end if
end program main