how to show diff between two row in another column in obiee

Hi ,

I have Below is the requirement :

	Y1	Y2
X1	100	0
X2	200	100
X3	400	200

FORMULA

Y2X1= 0

Y2X2=Y1X2 –Y1X1

Y2X3=Y1X3 – Y1X2

1.      Y3 is calculated column

2.      first row of y3 should always be 0

3.      2nd row onwards ex : 2^nd row : Y2X2=Y1X2 –Y1X1  3^rd row : Y2X3=Y1X3 – Y1X2

Note : All columns are associated with time period .

Any help would be greatly appreciated.

Thanks,

Manish Kumar