How does a normal calculator work?
You would begin the program with inputting the first number, (INPUT num1), then deciding if you have your pre-conditional loops and have inputting the second number inside them, to mimic a real life calculator, or inputting the second number.
To decide which function is used, i'd also control this with a string input (INPUT numfunction). To show the working out, IF numfunction == 'subtraction':
On a side note, i can almost guarantee that some syntax from Python 3 has made its way into this, but yeah.