First non-repeating letter

Problem

A quiz needs the first letter that appears only once in a word. Read one word and print that letter. If every letter repeats, print None.

Example

Input

swiss

Output

The letter s appears 3 times, w only once.

Try the problem first. Open one hint at a time.

Hint 1

A letter is unique when the whole word contains it exactly once. The method count() tells you that.

Hint 2

Check the letters from left to right and stop at the first unique one.

Write your Python code, run it on the examples and submit it to hidden tests.