[Python] Regular expression for splitting mentions of two years appearing altogether

I have the following case, where in my string I have improperly formatted mentions of the form "(19561958)" that I would like to split into "(1956-1958)". The regular expression that I tried is:

import re
a = "(19561958)"
re.sub(r"(\d\d\d\d\d\d\d\d)", r"\1-", a)


but this returns me "(1956195". How can I achieve my purpose?

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