Hi, I am new to PL/SQL, I'm trying to execute the commands that I learned at the course.

VARIABLE area NUMBER
DECLARE
  radius NUMBER(2) := &s_radius;
  pi CONSTANT NUMBER := 3.14;
BEGIN
  :area := pi * radius * radius;
END;

I understand that I can run this using SqlPlus, but I remember my teacher was running this from the web browser using Application Express. I try to run the same commands there, at HOME >SQL>SQL Commands, but I keep getting the error

"ORA-00900: invalid SQL statement"

.
Can you help me run it in Application Express or point me to a way where I can use an editor to run these course exercises?
Thanks!