Write a function, called fun, to calculate the factorial of a non$-$negative integer. Note that the factorial of an integer is defined as

$!=\backslash$times $(1)\backslash$times $(2)\backslash$times $\backslash$times $2\backslash$times $1$

n

$!$

$=$

n

$\backslash$times

$($

n

$$

$1$

$)$

$\backslash$times

$($

n

$$

$2$

$)$

$\backslash$times

$$

$\backslash$times

$2$

$\backslash$times

$1$

for $>=1$

n

$>=$

$1$

$,$ and $0!=1$

$0$

$!$

$=$

$1$

$.$ Make sure that an error message 'The input is not valid.$\text{'}$ is printed if $<0$

n

$<$

$0$

$.$