SELECT "GDP (billion US$)" FROM table_17980 WHERE "Area (km\u00b2)" = '83871'
What is the gdp of the country with an area of 83871 (km2)?
CREATE TABLE table_17980 (
    "Member countries" text,
    "Population" real,
    "Area (km\u00b2)" real,
    "GDP (billion US$)" text,
    "GDP per capita (US$)" real
)