##</code><code>##

use strict;
use warnings;

my %h;
while (<DATA>) {
    chomp;
    ($a,$b) = split;
    if ($a eq 'a') {
        $h{a} ||= $b;
    } elsif ($a eq 'b') {
        print $b - $h{a}, $/;
        %h = ();
    }
}

__DATA__
a 123
a 125
b 127
a 129
a 130
b 131
a 132
b 133

##</code><code>##


##</code><code>##