#include <bits/stdc++.h>
using namespace std;
template <class T, class U>
inline void chmin(T &t, U f) {
  if (t > f) t = f;
}
template <class T, class U>
inline void chmax(T &t, U f) {
  if (t < f) t = f;
}
int n, m, v, i, cnt = 0;
void solve() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  long long alp[30] = {0};
  for (int i = (int)(0); i < (int)(s.size()); i++) alp[s[i] - 'a']++;
  for (int i = (int)(0); i < (int)(30); i++) alp[i] %= 2;
  for (int i = (int)(0); i < (int)(30); i++) {
    if (alp[i] == 1) cnt++;
  }
  if (cnt == 0)
    cout << 1 << endl;
  else
    cout << cnt << endl;
}
int main() {
  solve();
  return 0;
}
