#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-8;
const int INF = (1 << 30) - 1;
const int mod = (int)(1e9 + 7);
char s[200010];
int num[27];
int N[26];
int main() {
  scanf("%s", s + 1);
  int len = strlen(s + 1);
  int ans = 0;
  for (int i = len; i >= 1; --i) {
    num[s[i] - 'a']++;
  }
  int odd = 0, cnt = 0;
  for (int i = 1; i <= len; ++i) {
    int id = s[i] - 'a';
    if (num[id] == 1) {
      cnt++;
      if (cnt == 2) {
        odd = 0;
        for (int j = 0; j < 26; ++j) odd += N[j] % 2;
        ans += max(1, odd);
        memset(N, 0, sizeof(N));
        cnt = 1;
      }
    }
    N[id]++;
  }
  odd = 0;
  for (int j = 0; j < 26; ++j) odd += N[j] % 2;
  ans += max(1, odd);
  printf("%d\n", max(1, ans));
  return 0;
}
