#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  int a[26] = {0};
  for (int i = 0; i < (int)S.size(); i++) {
    a[S[i] - 'a']++;
  }
  int count = 0;
  for (int i = 0; i < 26; i++) {
    if (a[i] % 2 == 1) count++;
  }
  if (count == 0) {
    cout << 1 << endl;
  } else {
    cout << count << endl;
  }
}
