#include <bits/stdc++.h>
using namespace std;
const long long inf = (long long)1e14;
const long long N = (long long)1e3 + 77;
const long long mod = (long long)1e9 + 7;
int n, m, k, x, y, ans;
string s;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> s;
  n = (int)s.size();
  map<char, int> mp;
  for (int i = 0; i < (int)(n); ++i) {
    mp[s[i]]++;
  }
  for (auto x : mp) {
    if (x.second & 1) ans++;
  }
  cout << max(1, ans) << endl;
  return 0;
}
