Problem Statement

You are given a string $S$ consisting of lowercase English letters. Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in $S$. If every lowercase English letter occurs in $S$, print None instead.

Constraints

• $1 \leq |S| \leq 10^5$ ($|S|$ is the length of string $S$.)
• $S$ consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

S

Output

Print the lexicographically smallest lowercase English letter that does not occur in $S$. If every lowercase English letter occurs in $S$, print None instead.