I did the same solution, but I get an error, what is the problem?

PL\SQL

DECLARE a NUMBER : =0; b NUMBER : =0; operation CHAR (1):= NULL; result NUMBER :=0; answer CHAR(3) : BEGIN LOOP dbms_output.put_1ine(' Enter number 9 a: i) 10. a t=8a; dbms_output.put_linef' Enter number 11ba); 12b:=8b; dbms output.put Iine('Enter the operation (+ for addition, - for subtraction, * for multiplication, 13. for division): '); operation := UPPER (SUBSTR (TRIM (TO_CHAR (read_1ine (PI 14(1)),1,1)); 15 CASE operation WHEN ' + ' THEN result :=a+b; WHEN ' - ' THEN result :=ab; WHEN " " THEN result :=ab; WHEN " / ' THEN result : = a / b; ELSE dbms_output.put_1ine ('Invalid 20 Operation )i 21 END CASE; 22 dbms_output.put_line ('The answer is: a 23 || operation || b= | | result : dbms output, put_line("Do you want to the operations (Subtraction/Addition/Multiplication/ 25 again?'); answer := UPPER ( SUBSTR (TRIM (TO_CHAR (read_line (PP) 26(1))),1,1)); 27. EXIT WHEN answer 1= 'YES'; 28 END LOOP; 29. END; TheN resutt :=abi TheN result i=a b '/. THEN result t =a a b SE dbas_output,put_Hine('Invalid Operation'); CASE; _output,put_Line("The answer is a " operation " b= " I1 resutt); 5_output, put_ine('Do you want to do the operations (Subtraction/Addition/Muttiplication/oivision) again?'); wer t= UPPER(SUasTR(TRIM LOAE: LOOP Error at Line 14/40: ORA-06550 tine 24, cotuan 4e: PL5-b02011 Identifier 'READ_LTe' aust be dectared. OMA-e65121 at "SYS. WW__OBAS_SOL_APEX_220200", tise 28 olta-065501 tine 14, cotuan it PL/SoL: stateeent ignored ofA-065se: line 24, colues 37i PL5-00201i identifier. 'AEAO_LtaE' must be dectared oft-265501 tine 24, coluen ir PL/SoL: Statesent ignered oMA-66512: at "SYS.DAHes SYS_SOL, Line 1650 1. Declane 2. a nubeta in ej 3. b mireca in ei