#include <bits/stdc++.h>
signed main() {
  std::string s;
  std::cin >> s;
  std::map<char, long long> count;
  for (auto i : s) count[i]++;
  long long min = 0;
  for (auto i : count) {
    if (i.second % 2 == 1) min++;
  }
  std::cout << (std::max(min, 1ll)) << '\n';
  return 0;
}
