Shahata Calculator play online

Calculator magic!
Step 1: Type a number you'd like to magically predict and save it by clicking "%".
Step 2: Write the predicted number on a piece of paper and give it to the crowd for safe keeping.
Step 3: Do whatever calculation you want by asking the crowd for random numbers using "+", "*" and "/" operations.
Step 4: Click the "-" button and tell the crowd you are going to blindly type a number in without even looking.
The trick: No matter what digits you click, the calculator will actually type the digits of the number that needs to be subtracted in order to reach your predicted number.
Step 5: Amaze the crowd by clicking "=" and showing that the result is actually the same as the number you gave then in the beginning of the trick :).
Step 6: Press "AC" and then "%" in order to disable the magical "-" if someone is suspecting it does something special :D.