       n     time (ms)       ratio        n(n+1)/2-1
    5000          4.10           -          12502499
   10000         16.09        3.93          50004999
   20000         64.06        3.98         200009999
   40000        256.48        4.00         800019999
   80000       1034.66        4.03        3200039999
