How can I merge two Python dictionaries in a single expression, adding up the elements with the same keys in both dicts?

Question

This is very similar to How to merge two Python dictionaries in a single expression?

However if my dicts are:

x = {'a':1, 'b': 2}
y = {'b':10, 'c': 11}

and my 'merged' dict is z

Then I would want:

print(z)

to output:

{'a': 1, 'b': 12, 'c': 11}

(Whereas in the linked question z's b key would have a value of 10)