#include <bits/stdc++.h>
using namespace std;
int main(void) {
  string s;
  cin >> s;
  int num[26] = {0};
  for (int i = 0; i < s.length(); i++) {
    num[s[i] - 'a']++;
  }
  int ki = 0;
  int gu = 0;
  for (int i = 0; i < 26; i++) {
    if (num[i] % 2 == 0)
      gu += num[i] / 2;
    else {
      gu += num[i] / 2;
      ki++;
    }
  }
  if (ki == 0) {
    cout << 1 << endl;
  } else {
    if (ki <= gu) {
      cout << ki << endl;
    } else {
      cout << ki << endl;
    }
  }
  return 0;
}
