P1836 Sum of Page Digits

A book’s page numbers are the consecutive integers from $1\sim n$: $1,2,3,\cdots,n$. Please compute the sum of all individual digits across all page numbers; for example, for page $123$, its sum is $ 1+2+3=6$.

A single integer $n$ on one line.

One line containing the sum of all individual digits.

Constraints: $1\le n\le 10^9$. Translated by ChatGPT 5