#include <bits/stdc++.h>
using namespace std;
long long dx[4] = {1, 0, -1, 0};
long long dy[4] = {0, -1, 0, 1};
template <class T>
void inputVector(vector<T>& v, long long n) {
  v.resize(n);
  for (long long i = 0; i < v.size(); i++) cin >> v[i];
}
long long cnt[30];
signed main() {
  string S;
  cin >> S;
  for (long long i = 0; i < (S.size()); i++) {
    cnt[S[i] - 'a']++;
  }
  long long ret = 0;
  for (long long i = 0; i < (30); i++) {
    ret += cnt[i] % 2;
  }
  cout << max(1ll, ret) << endl;
  return 0;
}
