sub f3 {
    my $n    = shift;

    my $t3   =  3 * int($n/3) * int($n/3 + 1) / 2;
    my $t5   =  5 * int($n/5) * int($n/5 + 1) / 2;
    my $t15  = 15 * int($n/15) * int($n/15 + 1) / 2;

    return $t3 + $t5 - $t15;
}