With April 1986 as the introduced, what is the least clock speed (MHz)?
CREATE TABLE "list_of_macintosh_models_grouped_by_cpu_" (
  "processor" text,
  "model" text,
  "clock_speed_m_hz" real,
  "l1_cache_bytes" text,
  "introduced" text,
  "discontinued" text
);
SELECT MIN("clock_speed_m_hz") FROM "list_of_macintosh_models_grouped_by_cpu_" WHERE "introduced"='april 1986';