#include <bits/stdc++.h>
using namespace std;
inline void output() { cerr << endl; }
template <typename First, typename... Rest>
inline void output(const First& first, const Rest&... rest) {
  cerr << first << " ";
  output(rest...);
}
using ll = long long;
using Pii = pair<long long, long long>;
using TUPLE = tuple<long long, long long, long long>;
using vi = vector<long long>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
const long long inf = 1ll << 60;
const long long mod = 1e9 + 7;
using Graph = vector<vector<long long>>;
signed main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  string s;
  cin >> s;
  map<char, long long> mp;
  for (long long i = 0; i < (s.size()); i++) {
    mp[s[i]]++;
  }
  long long ans = 0;
  for (auto p : mp) {
    if (p.second % 2) {
      ans++;
    }
  }
  cout << max(1ll, ans) << endl;
}
