First Unique Character Finder

Given a string s, your task is to find the first character that appears exactly once in the string. If no such character exists, output the underscore character '_'.

You need to process the string in a single pass (after counting the frequency of every character) to identify the first unique character. Formally, if we define \( f(c) \) as the number of occurrences of character \( c \) in \( s \), find the smallest index \( i \) such that \( f(s_i)=1 \). If no such \( i \) exists, output \( '_' \).

inputFormat

The input consists of a single line containing the string s (which may include any printable characters).

outputFormat

Output a single character — the first unique character in s. If there is no unique character, output '_'.

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