What is the sum of the heights in meters for the commerzbank tower built after 1984?
CREATE TABLE "tallest_completed_buildings" (
  "name" text,
  "height_m" real,
  "height_ft" real,
  "location" text,
  "year_built" real
);
SELECT SUM("height_m") FROM "tallest_completed_buildings" WHERE "year_built">1984 AND "name"='commerzbank tower';