说明： This program takes a positive integer from the user and computes factorial using for loop.
Since the factorial of a number may be very large, the type of factorial variable is declared as unsigned long long.
If the user enters negative number, the program displays error message.
文件列表：
factorial-number.jpg, 21844 , 2018-03-17
New Text Document.txt, 517 , 2018-03-17
