#include <bits/stdc++.h>
using namespace std;
using namespace std;
int main() {
  string s;
  cin >> s;
  int f[26] = {};
  for (long long i = 0; i < s.size(); i++) {
    f[s[i] - 'a']++;
  }
  int a = 0;
  for (long long i = 0; i < 26; i++) {
    if (f[i] % 2 != 0) a++;
  }
  if (a == 0) a++;
  cout << a << endl;
}
