#!/usr/bin/perl
use v5.35.0;
use List::Util qw(sum);

my $map_width;
my @map;
my @trailheads;
while (<>) {
	$map_width ||= length;
	for (split //) {
		push @map, /\d/? 0+$_ : undef;
		push @trailheads, $#map if /0/;
	}
}

{
	my @memo;
	sub rating($location) {
		return $memo[$location] if defined $memo[$location];

		my $rating = $map[$location] == 9;
		unless ($rating) {
			my $uphill = $map[$location] + 1;
			for my $offset (-$map_width, 1, $map_width, -1) {
				my $neighbour = $location + $offset;
				next unless 0 <= $neighbour < @map;
				next unless defined $map[$neighbour];
				next unless $map[$neighbour] == $uphill;
				$rating += rating($neighbour);
			}
		}
		return $memo[$location] = $rating;
	}
}

say sum map {rating($_)} @trailheads;