How to add values of 1st row to 2nd row and so on and show the sum in a different column

I have count of VMs per month and I want to show the count increment in a new column, for ex _time Monthly count 2014-06 49 2014-07 300 2014-08 5 2014-09 97 I want to show the incremental sum in a new column as this - _time Monthly count Incremental Sum 2014-06 49 49 2014-07 300 349 2014-08 5 354 2014-09 97 471