Calculate cost per month inlcuding cost of former months

In Row 2 there are the number of months a certain cost should be calculated (number changes every month)
In Row 3 is the cost at that particular month (cost changes every month)

I want to calculate the cost for every month but also taking also taken account the costs of former months

For example in the 1st month the costs are 10 and these are for 5 months, so in the next 5 months 10 cost should be calculated
In the 2nd month the costs are also 10 but these are for 4 months, so in the next 4 months these costs should be calculated

I also added an example which will make the question a lot more clear!

I hope somebody can help me to simplify my excel sheet!

Thanks!!

Hi
I have individual 12 formulae for copying across to right.

B5:

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In C6:

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In D7:

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and so on

Copy to left as needed

Regards