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?

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