SQL> ed
Wrote file afiedt.buf

  1  declare
  2     first_num       number:= &first_num;
  3     sec_num         number:= &sec_num;
  4     result          number;
  5  begin
  6     result  := first_num + sec_num;
  7     dbms_output.put_line('result is '||result);
  8* end;
SQL> set serveroutput on
SQL> /
Enter value for first_num: 10
old   2:        first_num       number:= &first_num;
new   2:        first_num       number:= 10;
Enter value for sec_num: 20
old   3:        sec_num         number:= &sec_num;
new   3:        sec_num         number:= 20;
result is 30

PL/SQL procedure successfully completed.