Most Frequent Word

Given a string s, find the most frequently occurring word. The word matching is case-insensitive. In case of ties (i.e. multiple words with the same maximal frequency), choose the word that appears first in the string.

Formally, let the input string be represented as a sequence of words \( w_1, w_2, \dots, w_n \). For each word \( w \), define its frequency as \( f(w) \). The answer is the word \( w^* \) such that \[ w^* = \arg\max_{w \in \{w_1,\dots,w_n\}} f(w), \] with ties broken by the order of appearance in the input.

If the input string is empty, output None.

inputFormat

The input consists of a single line, which is a string containing words separated by spaces. The string can be empty.

outputFormat

Output a single line containing the most frequent word (in lowercase). If the input string is empty, output None.

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