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

template <class T> void chmin(T &a, const T &b) {
  if (b < a)
    a = b;
}

using usize = std::size_t;

int main() {
  std::string s;
  std::cin >> s;

  usize ans = std::numeric_limits<usize>::max();

  std::vector<bool> parity(26, false);
  for (const auto e : s) {
    parity[e - 'a'].flip();
  }

  std::cout << std::count(parity.begin(), parity.end(), true) / 2 << std::endl;

  return 0;
}

