PROGRAMMING:Finding the factorial of n
This problem requires the preparation of procedures to calculate the factorial of $$n $.
###Input format:
Input gives a non negative integer $$n $$($$0 / Le n / Le 21 $$) in one line.
###Output format:
In one line, output the factorial value $$f $$in the format of "product = f". Note that there is a space on the left and right of the equal sign. The calculation results should not exceed the double precision range.
###Input example:
```in
five
```
###Output example:
```out
product = 120
```
answer：If there is no answer, please comment
###Input format:
Input gives a non negative integer $$n $$($$0 / Le n / Le 21 $$) in one line.
###Output format:
In one line, output the factorial value $$f $$in the format of "product = f". Note that there is a space on the left and right of the equal sign. The calculation results should not exceed the double precision range.
###Input example:
```in
five
```
###Output example:
```out
product = 120
```
answer：If there is no answer, please comment