HOW TO ADD VALUE FROM FIRST ROW TO SECOND ROW

Oracle OPM

Version 12.2.10

hello all

i need help to capture the value from first row to second row for example

fist month jan value is 10

second month feb value is 20

result should be as

jan 10 frist month as it is

feb 30 jan+feb like wise

query

SELECT R.BILL_TO_CUSTOMER_ID, TO_NUMBER (TO_CHAR (TRX_DATE, 'MM')) MNTH

,to_char(trx_date,'MON-YY') MONTH_YR

,to_char(trx_date,'YYYY') YEAR

,SUM (EXTENDED_AMOUNT) AMOUNT

FROM RA_CUSTOMER_TRX_ALL R, RA_CUSTOMER_TRX_LINES_ALL RTL

WHERE   R.CUSTOMER_TRX_ID = RTL.CUSTOMER_TRX_ID

AND R.ORG_ID = RTL.ORG_ID