The logic source towards the middle of the top edge of the screen controls whether it's in addition or subtraction mode. Low means addition and high means subtraction. The decimal point is for carry out and is only available for addition, and the minus sign means negative output and is only available for subtraction. If it gives a wrong answer that's just because it takes up to 5 seconds to finish computing and while it's doing that the output can give just about any answer no matter how wildly wrong.
