$7.$ The triple factorial of a non$-$negative integer n$,$ denoted by n$!!!,$ is the product of positive integers less than or equal to n$,$ where:

As an example:

$20!!!=20$ x $(20-3)$ x $(20-6)$ x $(20-9)$ x $(20-12)$ x $(20-15)$ x $(20-18)=$

$=20$ x $17$ x $14$ x $11$ x $8$ x $5$ x $2$

$=4188800$

Write a C$++$ Program to calculate the triple factorial of a positive number entered by the user. The number cannot be greater than $20.$ Validate your input.