What is the lowest density of serravalle scrivia?
CREATE TABLE "communes" (
  "rank" text,
  "city" text,
  "population" real,
  "area_km_2" real,
  "density_inhabitants_km_2" real,
  "altitude_mslm" real
);
SELECT MIN("density_inhabitants_km_2") FROM "communes" WHERE "city"='serravalle scrivia';