#include <bits/stdc++.h>
using namespace std;
template <class T>
void chmin(T& a, T const& b) {
  if (b < a) a = b;
}
using ll = long long;
const ll mod = LLONG_MAX;
int ans;
int how[27];
int main() {
  string s;
  cin >> s;
  for (int i = 0; i < 26; i++) {
    char alpha = 'a';
    char temp = alpha + i;
    for (int j = 0; j < s.size(); j++) {
      if (s[j] == temp) how[i]++;
    }
  }
  for (int i = 0; i < 26; i++) {
    if (how[i] % 2 != 0) ans++;
  }
  if (ans != 0) {
    cout << ans << endl;
  } else {
    cout << '1' << endl;
  }
}
