z-ram

Little Z has designed a new RAM chip. The memory on the chip can be viewed as a 20 by 20 matrix. Each value can be either 0 or 1.

In order to read memory, a user must select a row and a column. If the user selects multiple rows and columns, then he can read all the values that are on both selected rows and selected columns. For example if columns 1 and 2 are selected and rows 1 and 2 are selected, then the user can read values at (1,1), (1,2), (2,1) and (2,2).

Mister Little Z asked you the following question: Assume you can select n (2 <= n <= 40) rows or columns, what is the largest amount of 1s (ones) you can read