$ perl -e 'printf " %.1f => %.0f\n", $_, $_ for qw  / .5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5/;'
 0.5 => 0
 1.5 => 2
 2.5 => 2
 3.5 => 4
 4.5 => 4
 5.5 => 6
 6.5 => 6
 7.5 => 8
 8.5 => 8
 9.5 => 10