#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int vx[8] = {0, 1, 1, 1, 0, -1, -1, -1}, vy[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
inline bool check(int ux, int uy, int x, int y) {
  return (0 <= x and x < ux and 0 <= y and y < uy);
}
inline void init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}
map<char, int> mp;
int main() {
  string s;
  cin >> s;
  int n = s.size(), ans = 0;
  for (int i = (0); i < (n); ++i) mp[s[i]]++;
  for (char c = 'a'; c <= 'z'; ++c) {
    if (mp[c] % 2 == 1) {
      ans++;
    }
  }
  if (ans == 0) ans = 1;
  cout << ans << endl;
  return 0;
}
