sub smart {
    my $t=sum @_;
    if ($t%2) {
        my $i=($t-1)/2;
        ($i -= $_[$_])<0 and return $_ for 0..$#_;
    } else {
        my $acc=0;
        for my $i ($t/2-1, $t/2) {
            for (0..$#_) {
                ($acc += $_), last if ($i -= $_[$_])<0;
            }
        }
        return $acc/2;
    }
}