What's the sum of the population for the place simplified 上杭县 with an area smaller than 2,879?
CREATE TABLE "administration" (
  "english_name" text,
  "simplified" text,
  "traditional" text,
  "pinyin" text,
  "hakka" text,
  "area" real,
  "population" real,
  "density" real
);
SELECT SUM("population") FROM "administration" WHERE "simplified"='上杭县' AND "area"<'2,879';