# Really Unsecure Passwords

At ReallyUnsecurePassword.com, they demand that users have a password, which consists of the digits from 0 to 9, and is of length $$4$$ that uses exactly $$2$$ different digits. How many different passwords can there be?

Details and assumptions

$$0001$$ is a valid password, while $$3333$$ and $$0123$$ are not valid passwords because they do not use exactly $$2$$ different digits.

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