--- /dev/null
+#!/usr/bin/perl -w
+
+use strict;
+
+local $/;
+my @ads = sort { $a <=> $b } map { chomp; $_ } split /\n/, <>;
+
+my $prev = 0;
+my %diffs;
+for my $n (@ads) {
+ $diffs{ $n - $prev }++;
+ $prev = $n;
+}
+$diffs{3}++;
+
+print $diffs{1} * $diffs{3}, "\n";