#include <bits/stdc++.h>
const int kMaxLen = 3e5;
int count[30];
char s[kMaxLen];
int main() {
  scanf("%s", s);
  int len = std::strlen(s);
  for (int i = 0; i < len; i++) {
    count[s[i] - 'a']++;
  }
  int ans = 0;
  for (int i = 0; i < 26; i++) {
    if (count[i] % 2 == 1) {
      ans++;
    }
  }
  printf("%d\n", std::max(ans, 1));
  return 0;
}
