Error while substracting a smaller number from a bigger number.

[divya-gh]

Describe the bug
An Error was thrown when the result of the substation was a -ve number.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Calculator - calcy.py'
2. Open in Visual studio.
3. Run the script and When prompted to 'Enter Equation, type '2-3'(without quotes) and Enter
4. Result is displayed in nextline
5. Enter
6. Error was thrown
   

Expected behavior
After the result, I would expect the user to enter another equation or quit.

Desktop (please complete the following information):

• OS: Windows
• Python 3.6
• IDE -Visual Studio