Design+Your+Own+Program+(Variables+and+Arithmetic+Operators)

Task
Can you come up with your own short program which:
 * has at least one variable
 * includes an arithmetic operator (remember an arithmetic operator can be: + - * / ^

Design

 * Write down the inputs, processes and outputs of your program
 * Write down the names of any variables you will use
 * Write down the steps which the program will carry out

Development

 * Write the program