S - is the sum of a square's sides, than that sum (S) is multiplied by 4 and as a result you get a perimeter of squares.
So S(n) - is a length of nth square side.

And you need to write a function that will calculte 4 * S = 4 * ( S(0) + S(1) + .... + S(n) )

An IntStream. See https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html#chars--