What is the sum of the serial numbers for narowal city?
CREATE TABLE "punjab" (
  "serial_no" real,
  "district" text,
  "headquartered_city" text,
  "city_population_2009" real,
  "city_area_km_2" real
);
SELECT SUM("serial_no") FROM "punjab" WHERE "headquartered_city"='narowal city';