ORA-06502: PL/SQL: character to number conversion error while using variable

I know its been asked a lot but didn’t know how to apply it in a variable;

this is my code

DECLARE

Main_time_elap varchar2(1000);
time_elap  varchar2(1000);
time_elape number(5);

BEGIN


            main_time_elap:=to_char(systimestamp,'HH24:MI:SS');

            time_elap:=to_char(systimestamp,'HH24:MI:SS');
            time_elape:=to_number(time_elap)-to_number(main_time_elap);
            --time_elape :=time_elap-main_time_elap ;


end;
/

>Solution :

It is much simpler:

DECLARE

Main_time_elap TIMESTAMP;
time_elap  TIMESTAMP;
time_elape INTERVAL DAY TO SECOND;
sec_elape NUMBER;

BEGIN
       main_time_elap := systimestamp;

       time_elap := systimestamp;
       time_elape := time_elap - main_time_elap;    
       sec_elape := EXTRACT(SECOND FROM time_elape);
end;
/