CREATE TABLE "department" (
"Department_ID" int,
"Name" text,
"Creation" text,
"Ranking" int,
"Budget_in_Billions" real,
"Num_Employees" real,
PRIMARY KEY ("Department_ID")
)

CREATE TABLE "head" (
"head_ID" int,
"name" text,
"born_state" text,
"age" real,
PRIMARY KEY ("head_ID")
)

CREATE TABLE "management" (
"department_ID" int,
"head_ID" int,
"temporary_acting" text,
PRIMARY KEY ("Department_ID","head_ID"),
FOREIGN KEY ("Department_ID") REFERENCES `department`("Department_ID"),
FOREIGN KEY ("head_ID") REFERENCES `head`("head_ID")
)
SELECT count(*) FROM department WHERE department_id NOT IN (SELECT department_id FROM management);
How many departments are led by heads who are not mentioned?