#include <bits/stdc++.h>
bool debug = true;
std::string s;
std::vector<int> table;
int main() {
  std::cin >> s;
  table.resize(26, 0);
  for (auto(it) = (s).begin(), _END = (s).end(); it != _END; ++it) {
    table[(int)(*it - 'a')]++;
  }
  int odd = 0;
  for (auto(it) = (table).begin(), _END = (table).end(); it != _END; ++it) {
    if (*it > 0) {
      if (*it % 2 != 0) {
        odd++;
      }
    }
  }
  std::cout << std::max(1, odd) << std::endl;
}
