sub nneighbours {
    die "Bad Input Matrix: must be even" if @_ % 2; 
    my @mat = @_;
    my $sum = 0;
    my $limit = $#mat;
    for(my $i = 0; $i < $limit; $i += 2){
        my $nearest;
        for(my $j = 0; $j < $limit; $j += 2){
            next if $i == $j;
            my $dist = sqrt(($mat[$i] - $mat[$j]) ** 2 +
                            ($mat[$i+1] - $mat[$j+1]) ** 2);
            $nearest = defined $nearest?
                       ($nearest > $dist ? $dist : $nearest)
                       : $dist;
        }
        $sum += $nearest;
    }
    return $sum;
}