How to write a function in python 3 that takes two dictionaries b1 and b2 and returns a new dictionary b3 which is the combination of b1 and b2 such that the function adds values for common keys....

How to write a function in python 3 that takes two dictionaries b1 and b2 and returns a new dictionary b3 which is the combination of b1 and b2 such that the function adds values for common keys.

Input:

b1 = {'a':1,'b':10,'c':3}

b2 = {'a':2,'b':11,'d':4}

Output:

b3 = {'a':3,'b':21,'c':3,'d':4}