You can check which button was pressed in the conditional branch part.
Now, you can use a common event, two variables and one switch. Set the first variable to the number corresponding to the "correct" key, check the button pressing and set the other variable according to which button was pressed. Now compare both variables, if they are the same, the correct button was pressed. If not, the wrong one was pressed. The just turn on/off the switch then. All you need to do is to check the switch in the normal event.
I hope I explained that well. (>.<)