  The running time of a quantum walk search algorithm depends on both the
structure of the search space (graph) and the configuration of marked
locations. While the first dependence have been studied in a number of papers,
the second dependence remains mostly unstudied.
  We study search by quantum walks on two-dimensional grid using the algorithm
of Ambainis, Kempe and Rivosh [AKR05]. The original paper analyses one and two
marked location cases only. We move beyond two marked locations and study the
behaviour of the algorithm for an arbitrary configuration of marked locations.
  In this paper we prove two results showing the importance of how the marked
locations are arranged. First, we present two placements of $k$ marked
locations for which the number of steps of the algorithm differs by
$\Omega(\sqrt{k})$ factor. Second, we present two configurations of $k$ and
$\sqrt{k}$ marked locations having the same number of steps and probability to
find a marked location.
