If one writes the natural numbers from 1 to 9, the sum of the digits used to write these numbers is 45. If one writes the natural numbers from 1 to 15, the sum of the digits used to write these numbers is 66. If one writes the natural numbers from 1 to 99, the sum of the digits used to write these numbers is 900... Is there a formula to compute the sum of the digits used to write the natural numbers from 1 to n?