 Decagonal number

A decagonal number is a figurate number that represents a decagon, which is a ten-sided polygon. The \(n\)-th decagonal number can be calculated using the formula: \[ D_n = \frac{n(4n - 3)}{2} \] where \(D_n\) is the \(n\)-th decagonal number and \(n\) is a positive integer.