#include <bits/stdc++.h>
using namespace std;
const int MAX = 2 * 100005;
char a[MAX];
int b[26];
int main() {
  cin >> a;
  int len = strlen(a);
  for (int i = 0; i < len; i++) {
    b[a[i] - 'a']++;
  }
  int sum1 = 0, sum2 = 0, sum3 = 0;
  for (int i = 0; i < 26; i++) {
    if (b[i] == 0)
      sum3++;
    else if (b[i] % 2 == 0)
      sum2++;
    else
      sum1++;
  }
  if (sum1 == 0)
    cout << 1 << endl;
  else
    cout << sum1 << endl;
  return 0;
}
