Which ERP W is the lowest one that has a Frequency MHz of 91.3?
CREATE TABLE "full_powered_stations" (
  "call_sign" text,
  "frequency_m_hz" real,
  "city_of_license" text,
  "erp_w" real,
  "class" text,
  "fcc_info" text
);
SELECT MIN("erp_w") FROM "full_powered_stations" WHERE "frequency_m_hz"=91.3;