Population has outstripped the supply of freshwater, usually from rainfall. The northern atolls get 50 inches (1,300 mm) of rainfall annually; the southern atolls about twice that. The threat of drought is commonplace throughout the island chains.

Answer this question, if possible (if impossible, reply "unanswerable"): How many millimeters of rain do the northern atolls of the Marshall Islands receive?
1,300