Drop down value to change sum of another box

Hello.

I am trying to create a simple function for my drop down list.
Right now you can select "Yes" or "No" in the drop down list.

I would like that when you select Yes the sum of a field changes (H18).

The background of this is that i am setting up an sheet to calculate monthly interest from different loans i have.

This drop down list should represent if i want to make monthly payments on the actual loan itself or only pay the interest.

Column called: Interest free
Value: YES (Equals no interest)
Value: NO (Pay interest)

Field H18 is the sum of H4 to H12 which are the interests from the loans.
Field H18 is also the sum of i4 to i12 which are the payments on the loans.

I would like that when i Select Yes (no payment on the loan) the H18 is substracted with the sum in the correct "i" field.

I hope that i explained this in the correct way,

I can try and upload the sheet so you can have a look at it.

Thanks for the help.