use strict;
use warnings;
use List::Util qw/max/;
my $graphspec = '1 -> 2 2 -> 0 3 -> 4 4 -> 2 5 -> 10 6 -> 10 7 -> 9 8 -> 9
9 -> 6 10 -> 4 11 -> 10 12 -> 10 13 -> 12 14 -> 10 15 -> 2';

my %graph;
while ($graphspec =~ /(\d+)\s+->\s+(\d+)/g) {
    $graph{$1} = $2;
}

sub depth {
    return max map { $_and 1 + depth($graph{$_}) } @_;
}

print depth(keys %graph);