#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[26];
int main() {
  scanf("%s", s + 1);
  int len = strlen(s + 1);
  int ans = 0;
  for (int i = 1; i <= len; ++i) {
    num[s[i] - 'a']++;
  }
  for (int i = 0; i < 26; ++i) {
    if (num[i] % 2) ans++;
  }
  printf("%d\n", max(1, ans));
  return 0;
}
