18. (5 points)
"A narcissistic number" refers to a class of numbers where the sum of the cubes of each digit equals the original number, for example, $370,3^{3}+7^{3}+0^{3}=27+343+0=370$. Adding the cubes of the digits of a number to get a new number is called one operation. Starting from 645 and repeatedly performing this operation, the final narcissistic number obtained is $\qquad$
$153$