# ([1], romanian national olympiad, 10th grade, 2000) How many ways can you tile a 3xn rectangle with dominoes.
# (romanian IOI selection, 1999)
<tex>

  for i1 = 1,n\newline
      for i2 = i1,n\newline
          for i3 = i2,n\newline
               …\newline
               for ik = ik - 1,n\newline
                   print ‘*’\newline

  for i1 = 1,n
      for i2 = i1,n
          for i3 = i2,n
               …
               for ik = ik - 1,n
                   print ‘*’

</tex>
How many stars will be printed for a given n and k.
# ([5], 'acm.sgu.ru':http://acm.sgu.ru/problem.php?contest=0&problem=222) How many ways can k rooks be placed on a nxn chessboard so that they don’t attack each other.