#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int solve(string& s) {
  int n = s.length();
  int hoge = 0, ans = 0;
  vector<int> cnt(26, 0);
  for (int i = 0; i < n; i++) {
    if (cnt[s[i] - 'a'] % 2 == 0) {
      hoge++;
    } else {
      hoge--;
    }
    cnt[s[i] - 'a']++;
  }
  return max(hoge, 1);
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  int ans = solve(s);
  reverse(s.begin(), s.end());
  ans = min(ans, solve(s));
  cout << ans << endl;
  return 0;
}
