Name the sum of frequecy with brand of exa fm
CREATE TABLE "fm_radio" (
  "frequency" real,
  "callsign" text,
  "brand" text,
  "format" text,
  "city_of_license" text,
  "website" text,
  "webcast" text
);
SELECT SUM("frequency") FROM "fm_radio" WHERE "brand"='exa fm';