#!perl -w
use strict;

chomp(my @list = <DATA>);

# FORE!!! 78 chars.
sub g {my%h=();my$n=pop;$h{abs($_-$n)}=$_ for@_;my@s=sort{$a<=>$b}keys%h;$h{shift@s};}

print g(@list,10);

__DATA__
17
1.4
18.2
7.9
12.2
12.5
1.1
7.8
18.3
20
6.7
6.9
18.1
1.5
17.7
16.6
1.2
1.3
17.5