Need Advice with creating formula

I wanted to make a formula in Excel
try to tell...

for example, I have a 5-storey house
comes to the ventilation duct
For each of these floors goes air duct, each floor its own duct
This means if the air ducts in the attic begins, then the 5th floor in the attic there are 5 air ducts
4 floor 4 air ducts
3rd floor 3
2nd floor 2
1st floor 1
one runs the length of the air on one floor 3m
and I want to get the formula Excel
order by entering the number of floors and floor height is automatically calculated as the total duct length by all floors
this case it is 45 m
1st floor is 3m
2nd floor is 6m + 1st floor = 9m
formula.jpg
this how it looks like
first one is with 5 floors
second one is with 6 floors.

I want to create formula witch by changing floor count, changes result