sub string_distance {
  my ($a, $b) = @_;
  my @a = split //, $a;
  my @b = split //, $b;
  my $count = 0;
  for (my $i = 0; $i < length($a); $i++) {
    $count++ if ($a[$i] eq $b[$i]);
  }
  return $count;
}