Creating User Defined Exceptions

In Python, users can define such exceptions by creating a new class.
This exception class has to be derived, either directly or indirectly, from Exception class.
Most of the built-in exceptions are also derived form this class.
Example:

class NegativeNumberException(Exception):
    pass

while(True):
    try:
        a=int(input("Input first no:"))
        b=int(input("Input second no:"))
        if a

Output:

Input first no: 10
Input second no: -3
Negative numbers are not allowed!Try again

Input first no: 10
Input second no: 0
Please input non-zero denominator

Input first no: 10
Input second no: a
Please input integers only! Try again

Input first no: 10
Input second no: 5
Div is  2.0