use strict;
use warnings;

my %before = (
    a => ['c'],
    b => ['d','e'],
    c => ['l'],
    d => ['e','a'],
    e => ['c'],
    f => ['g','d'],
    g => ['c'],
    h => ['g','i'],
    i => [],
    j => ['c'],
    k => [],
    l => [],
    n => ['c'],
    o => [],
);

my ($k, $m, %p, @order);
while (scalar keys %before) {
    for $k (keys %before) {
        $m = 1;
        for (@{$before{$k}}) {
            if (!exists $p{$_}) {
                $m = 0; last;
            }
        }
        if ($m) {
            push(@order, $k);
            $p{$k} = ();
            delete($before{$k});
        }
    }
}
@order = reverse @order;
print @order;