hi, i need help modelling a use case diagram from a topic, it will be in java GUI

Design a Calculator that

1.Allow user to key in a legitimate arithmetic statement that involves number, operator +, - and bracket '(' and ')' ;

2.When user press “Calculate” button, display result;

3.Some legitimate statement would be ((3+2)-4+2) (equals 3) and (-2+3)-(3-1) (equals -1);

4.You should NOT use a pre-existing function that just take in the statement as a parameter and returns the result but you should write the logic of parsing every character in your code.

5.Store the last statement and answer so it is displayed when user press the “Last calculation” button.

i have designed two use case diagrams using UML on netbeans 6.5.1, one of the use case i am not sure whether is it containing too much use cases etc, while the other is what i think could be too vague for the topic.i hope to get some feedback on whether the use case diagram are appropriate, thanks.