<image>As shown in the image, matchsticks are used to form squares with side lengths of 1, 2, and 3 matchsticks. If the side length is \( n \) matchsticks, and the number of matchsticks used to form the square is \( S \), then \( S = \) __ (expressed as an algebraic formula containing \( n \), where \( n \) is a positive integer). _