What is the smallest total produced with a model of M-420?
CREATE TABLE "4_axle_units" (
  "model" text,
  "build_date" text,
  "total_produced" real,
  "wheel_arrangement" text,
  "prime_mover" text,
  "power_output" text
);
SELECT MIN("total_produced") FROM "4_axle_units" WHERE "model"='m-420';