Given a string s of English letters, return the greatest English letter that appears as both a lowercase and an uppercase letter in s. The returned letter must be in uppercase. If no such letter exists, return an empty string "".

Letter $B$ is considered greater than letter $A$ if $B$ appears after $A$ in the English alphabet.

Constraints:

• $1 \leq$ s.length $\leq 1000$

• s consists of lowercase and uppercase English letters.

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