Calling the procedure at the same time

There is a button for calculating score in website.

Each time the user click this button, it will call the Oracle procedure to calculate score.

The process of calculating score spends almost 2 minutes.

The problem is that there are two users click this button in the meantime.

The second user will get a error message because the first one is still in process.

Is there any solution to limit the second user to click this button before the first one finished.