How much is the GDP for a population density of 254.7?
CREATE TABLE "demographics" (
  "country" text,
  "area_km" text,
  "population_2011_est" text,
  "population_density_per_km" text,
  "gdp_ppp_m_usd" text
);
SELECT "gdp_ppp_m_usd" FROM "demographics" WHERE "population_density_per_km"='254.7';