ans: Returns the answer of the previous calculation.
PI: Returns PI.
E: Returns E.
+: Adds 2 numbers.
-: Subtracts 2 numbers.
*: Multiplies 2 numbers.
/: Divides 2 numbers.
^: Returns A to the power of B.
sqrt: Returns the square root of the first number.
average: Returns the average of 2 numbers using (A+B)/2.