#include <bits/stdc++.h>
using namespace std;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = numeric_limits<int>::max() / 2;
const int NEG_INF = numeric_limits<int>::min() / 2;
const int MOD = 1e9 + 7;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  int len = s.length();
  vector<int> dp(len + 1);
  for (int i = 0; i <= len; i++) dp[i] = i;
  vector<vector<long long> > cnt(len + 1, vector<long long>(26, 0));
  for (int i = 1; i <= len; i++) {
    int idx = s[i - 1] - 'a';
    for (int j = 0; j < 26; j++) {
      cnt[i][j] = cnt[i - 1][j];
    }
    cnt[i][idx]++;
  }
  int tmp = 0;
  for (int i = 0; i <= 26; i++) {
    if (cnt[len][i] % 2) tmp++;
  }
  std::cout << max(1, tmp) << endl;
}
