Customize pricing depending on the length and width

[werksigna]

Im trying to sell posters that have custom sizes and would like to change pricing depending on the length and the width provided by the customer, how do I go about this in the admin page?

For example, a client chooses a poster, then adds the length and width, the price should vary depending on the length and width variables.

i.e
Price = price/length + price/width