#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>

/*
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*/


int main(void) {
	std::string s;

	std::cin >> s;

	int n_alpha[26] = { 0 };

	for (int i = 0; i < s.length(); i++)
	{
		n_alpha[s[i] - 'a']++;
	}

	if (s.length() % 2 == 1) {
		for (int i = 0; i < 26; i++)
		{
			if (n_alpha[i] % 2 == 1) {
				n_alpha[i]--;
				break;
			}
		}
	}

	int ans = 0, count = 0;

	for (int i = 0; i < 26; i++)
	{
		if (n_alpha[i] % 2 == 1) {
			if (count == 0) {
				count++;
			} else {
				count = 0;
				ans++;
			}
		}
	}

	std::cout << ans + count << std::endl;
	return 0;
}

Measured memory_bytes: 3136