#include <bits/stdc++.h>
using namespace std;
const long long MD = 1000000007;
int main() {
  long long num[26] = {};
  string str;
  cin >> str;
  long long len = str.length();
  for (long long i = 0; i < len; i++) {
    for (int j = 0; j < 26; j++) {
      if (str[i] == 'a' + j) {
        num[j]++;
        break;
      }
    }
  }
  long long answer = 1;
  long long odd = 0;
  for (int i = 0; i < 26; i++) {
    if (num[i] % 2 != 0) {
      odd++;
    }
  }
  if (odd > 1) {
    answer = answer + (odd - 1);
  }
  cout << answer;
  return 0;
}
