N    time (mS)
----------------------
      1200   21.3189
      1600   36.6598
      2000   57.9936
      2500   89.0848
      3000   125.687
      4000   211.541
      5000   356.552
      7000   651.038
     10000   1371.34
     12000   1961.99
     15000   2982.12
     20000   5446.31
     25000   8546.15