#include <bits/stdc++.h>
using namespace std;
char s[200005];
int cnt;
int f[50];
int main() {
  scanf("%s", s + 1);
  int n = strlen(s + 1);
  for (int i = 1; i <= n; i++) {
    f[s[i] - 'a' + 1] = f[s[i] - 'a' + 1] + 1;
  }
  int i, j;
  for (j = 1; j <= 26; j++) {
    if (f[j] % 2 == 1) cnt++;
  }
  if (cnt == 0) {
    cout << "1" << endl;
    return 0;
  }
  cout << cnt << endl;
}
