First Non-Repeating Character

Given a string s, your task is to find the first character that does not repeat anywhere in the string. If no such character exists, output an underscore '_'. For example, in the string "leetcode", the first non-repeating character is 'l', while in "aabbcc" every character repeats so the answer is '_'.

The problem requires reading input from standard input (stdin) and printing the result to standard output (stdout).

Note: The solution should use appropriate algorithms and data structures to achieve efficient performance.

Mathematically, if we define a function \( f(s) \) such that

[ f(s)= \begin{cases} c & \text{if } c \text{ is the first non-repeating character in } s, \ '_' & \text{if every character repeats.} \end{cases} ]

then your program should output \( f(s) \) after processing the given input string.

inputFormat

The input consists of a single line containing the string s. The string may include letters, digits, or other characters. It is guaranteed that the string will not exceed 10⁴ characters.

Input is given via standard input (stdin).

outputFormat

Output a single character, which is the first non-repeating character in the string s. If there is no such character, output an underscore '_'.

Output should be written to standard output (stdout).

## sample

leetcode

l