SQL & PL/SQL :: Cumulative Some Last 5 Rows?

I need to get the sum of next 5 rows that are order by one specific column .

Ex :

DROP TABLE TEST_5;

CREATE TABLE TEST_5 AS
SELECT TO_NUMBER(200 || ROWNUM) YEAR, ROWNUM TOT
FROM ALL_OBJECTS
WHERE ROWNUM < = 15 ;

SQL> select * from test_5 order by year;

YEAR TOT
---------- ----------
2001 1
2002 2
2003 3
2004 4
2005 5
2006 6
2007 7
2008 8
2009 9
20010 10
20011 11
20012 12
20013 13
20014 14
20015 15

15 rows selected

SQL>

Now Expected Results :

SQL> select * from test_5;

YEAR TOT Exp_res
---------- -----------------------
2001 1 1+2+3+4+5 = 15
2002 2 2+3+4+5+6 = 20
2003 3 3+4+5+6+7 = 25
2004 4
2005 5
2006 6
2007 7
2008 8
2009 9
20010 10 10+11+12+13+14
20011 11 11+12+13+14+15
20012 12 12+13+14+15
20013 13 13+14+15
20014 14 14+15
20015 15 15

15 rows selected

SQL>