#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<char, int>;
const ll inf = numeric_limits<ll>::max() / 3;
int main(void) {
  string s;
  cin >> s;
  int cnt[30] = {};
  for (int i = 0; i < (int)s.size(); ++i) {
    ++cnt[s[i] - 'a'];
  }
  int res = 0;
  for (int i = 0; i < 30; ++i) {
    if (cnt[i] % 2 == 1) ++res;
  }
  cout << 1 + max(res - 1, 0) << endl;
  return 0;
}
