Given a positive integer n, convert each digit of n into its English word representation and concatenate them together in order from left to right.

The digit mappings are as follows:

• $0 \rightarrow$ "zero"

• $1 \rightarrow$ "one"

• $2 \rightarrow$ "two"

• $3 \rightarrow$ "three"

• $4 \rightarrow$ "four"

• $5 \rightarrow$ "five"

• $6 \rightarrow$ "six"

• $7 \rightarrow$ "seven"

• $8 \rightarrow$ "eight"

• $9 \rightarrow$ "nine"

Return the number of unique characters that have an odd frequency in the concatenated string.

Constraints:

• $1 \le$ n $\le 10^9$

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