 This is a study using pembrolizumab MK-3475, KEYTRUDA for first-line treatment of participants with advancedunresectable inoperable or metastatic urothelial cancer who are ineligible for cisplatin-based therapy. The primary study objective is to determine the objective response rate ORR in all participants, in participants whose tumors rely on programmed cell death ligand 1 PD-L1 protein PD-L1-positive tumors, and in participants with strongly PD-L1-positive tumors. 