How to get this plsql output

Question

The below is my plsql1.sql file
set serveroutput on;
DECLARE
a NUMBER;
BEGIN              
Dbms_Output.put_line('Description 475');
Dbms_Output.put_line('Description 574');
a:=&a;
Dbms_Output.put_line('Value of a is '||a);
END;this is my log file
SQL> @plsql1.sql
  9  /
Enter value for a: 10
old   6: a:=&a;
new   6: a:=10;
Description 475                                                                
Description 574                                                                
Value of a is 10                                                               
PL/SQL procedure successfully completed.
SQL> spool offI want below output
Description 475                                                                
Description 574                                                                
Enter value for a: 10
old   6: a:=&a;
new   6: a:=10;
Value of a is 10                                                               
i.e., I want Description 475 and Description 574 to be printed first and then ask for value a and then print that line,
please help me,,

Answer 1

Or change it into two pl/sql blocks:
set serveroutput on;
BEGIN              
Dbms_Output.put_line('Description 475');
Dbms_Output.put_line('Description 574');
end;
DECLARE
a NUMBER;
a:=&a;
Dbms_Output.put_line('Value of a is '||a);
END;
/