AMO · 2027 · Grades 6-7 · Self-study SET 29 Question 6

A number has 2021 digits. Let P be the sum of its digits, Q the sum of the digits of P, and R the sum of the digits of Q. Each operation is one ordinary sum of digits, not a repeated digital-root calculation. Find the largest possible Q.

**Input:** Enter the integer only, without units.