15. Let $1 \times 2 \times 3 \times \ldots \times 2018$ be denoted as 2018!. When 2018! is divided by 3, 2018! can be divided by 3, resulting in a quotient; then divide this quotient by 3, ... continue dividing by 3 until the quotient cannot be divided by 3 anymore. In this process, 3 divides 2018! a total of $\qquad$ times.
Reference answer: 1004