#include <bits/stdc++.h>
using namespace std;
int cnt[100005][26];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  string s;
  cin >> s;
  memset(cnt, 0, sizeof(cnt));
  int ans = 0;
  for (int i = (0); i < (s.length()); ++i) {
    cnt[0][s[i] - 'a']++;
  }
  for (int i = (0); i < (26); ++i) {
    if (cnt[0][i] % 2 == 1) ans++;
  }
  if (ans = 0) ans = 1;
  cout << ans << endl;
  return 0;
}
