Factorial

**Specification: Factorial**
The factorial of a number is the product of all of the integers between 1 and that number. e.g. 4 factorial = 1 x 2 x 3 x 4

Write a program which asks for a number, and calculates the factorial of that number. The program should also validate the input by displaying an appropriate message if the number entered is less than 1 or greater than 12.

The Task
1. Draw an Input-Process-Output diagram 2. Draw a screen design, with labelled fields and buttons 3. Produce a pseudocode design 4. Write the program