Create trends from different dates

[Hauglaen]

Hi,

I have the following sales data:

Date	Sales product A	Sales product B	Sales product C
jan.20	100
feb.20	50	90
mar.20	60	60	110
apr.20	78	60	30
mai.20	60	50	30
jun.20			50

I want to create a first, second, third ect. sales month.
Rather than compare jan.20 with feb.20.

How can i creat a table like this:

Month	Total sales
1	300
2	140
3	150
4	178
5	60

Thanks!