If the patentee has not exploited his invention or has exploited it in an inadequate fashion unless justified with a legitimate excuse, for a period of four years from the application filing date or three years from the granting date whichever expires later, the Patent Office may grant a compulsory license to a third party upon an application submitted thereof, provided that the applicant for the compulsory license proves that he has exerted-over a reasonable period of time-efforts in order to obtain a contractual license of reasonable commercial conditions, or for reasonable financial compensation.