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Score : $200$ points

### 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.

### Sample Input 1

atcoderregularcontest

### Sample Output 1

b

The string atcoderregularcontest contains a, but does not contain b.

### Sample Input 2

abcdefghijklmnopqrstuvwxyz

### Sample Output 2

None

This string contains every lowercase English letter.

### Sample Input 3

fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg

d