How to get two times having 24 hour format and get the difference in Minutes

How to get two times having 24 hour format and get the difference in Minutes
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Execute the Function

CREATE OR REPLACE FUNCTION TIMDIFF(ATA IN VARCHAR, STA IN VARCHAR) 
 RETURN NUMBER IS  DIFFTIMEINMINS NUMBER(5);
  BEGIN 
    If ATA IS NULL OR STA IS NULL Then
        RETURN 0;
    End If;
    If ATA IS NOT NULL AND STA IS NOT NULL THEN
     BEGIN        
        SELECT DISTINCT ROUND(TO_NUMBER((TO_DATE(sysdate || ' ' || ''||ATA||'','DD-mon-yyyy HH24MI')-TO_DATE (sysdate || ' ' || ''||STA||'','DD-mon-yyyy HH24MI'))*1440)) DOS INTO DIFFTIMEINMINS  FROM DUAL;
        RETURN DIFFTIMEINMINS;
     END;
    END IF;
  END;
/

Execute the Query 

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Select Timdiff('1234','1235')from Dual;