When one wrongdoer can get others to share the cost

[(a)] The right of contribution exists among joint tortfeasors. [(b)] A joint tortfeasor is not entitled to a money judgment for contribution until the joint tortfeasor has by payment discharged the common liability or has paid more than the joint tortfeasor's pro rata share thereof. [(c)] A joint tortfeasor who enters into a settlement with the injured person is not entitled to recover contribution from another joint tortfeasor whose liability to the injured person is not extinguished by the settlement. [(d)] When there is such a disproportion of fault among joint tortfeasors as to render inequitable an equal distribution among them of the common liability by contribution, the relative degrees of fault of the joint tortfeasors shall be considered in determining their pro rata shares, subject to section 663-17.