Find the First Unique Character

Given a string s, your task is to find the first character that does not repeat in the string. If all characters repeat or if the string is empty, output None.

For example, consider the string swiss. The first non-repeating character is w because although s appears multiple times, w appears exactly once.

The solution should be efficient and read input from standard input (stdin) and produce output to standard output (stdout).

Formally, let the string be s and define the frequency function as:

\( f(c) = \text{number of occurrences of } c \text{ in } s \)

Find the smallest index \( i \) such that \( f(s_i) = 1 \), and output \( s_i \). If no such index exists, output None.

inputFormat

The input consists of a single line containing the string s.

Note that s can contain any characters and its length could be zero.

outputFormat

Output the first non-repeating character of the string. If there is no such character, output None.

## sample

swiss

w