#include <bits/stdc++.h>
using namespace std;
int num[27];
int main() {
  string s;
  cin >> s;
  int n = s.size();
  memset(num, 0, sizeof(num));
  for (int i = 0; i < n; i++) {
    num[s[i] - 'a']++;
  }
  int ans = 0, flag = 0;
  for (int i = 0; i < 26; i++) {
    if (num[i] % 2) {
      ans++;
    }
    if (num[i] / 2 > 0 && num[i] % 2 == 0) flag = 1;
  }
  if (ans == 0) ans++;
  cout << ans << endl;
}
