#include <bits/stdc++.h>
using namespace std;
int cnt[26];
int main() {
  string s;
  cin >> s;
  for (char c : s) {
    cnt[c - 'a']++;
  }
  int ret = 0;
  for (int i = 0; i < 26; i++) {
    if (cnt[i] % 2 == 1) ret++;
  }
  if (ret == 0) ret++;
  cout << ret << endl;
}
