PROGRAMMING:Function returns the factorial of an integer

Write a function to return the factorial of an integer, the main function input integer n, output n factorial value（ The calculation result does not exceed long long type
###Input example:
```in
five
```
###Output example:
```out
5!= one hundred and twenty
```
###Input example:
```in
twenty
```
###Output example:
```out
20!= 2432902008176640000
```







answer：If there is no answer, please comment