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

my $map_width;
my @map;
my @trailheads;
my @peaks;
while (<>) {
	$map_width ||= length;
	for (split //) {
		push @map, /\d/? 0+$_ : undef;
		push @trailheads, $#map if /0/;
		push @peaks, $#map if /9/;
	}
}

my @score = (0) x @map;
my @reached = (-1) x @map;
for my $peak (@peaks) {
	my @queue = $peak;
	while (@queue) {
		my $location = shift @queue;
		next if $reached[$location] == $peak;
		$reached[$location] = $peak;
		++$score[$location];

		my $downhill = $map[$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] == $downhill;
			push @queue, $neighbour;
		}
	}
}

say sum map {$score[$_]} @trailheads;