#include <bits/stdc++.h>
using namespace std;
inline int read() {
  char c;
  int s = 0, t = 1;
  while (!isdigit(c = getchar()))
    if (c == '-') t = -1;
  do {
    s = s * 10 + c - '0';
  } while (isdigit(c = getchar()));
  return s * t;
}
inline long long readl() {
  char c;
  long long s = 0;
  int t = 1;
  while (!isdigit(c = getchar()))
    if (c == '-') t = -1;
  do {
    s = s * 10 + c - '0';
  } while (isdigit(c = getchar()));
  return s * t;
}
int a[100010];
int main() {
  string s;
  cin >> s;
  int l = s.length();
  for (int i = 0; i <= l - 1; i++) a[(int)s[i] - 96] = 1 - a[(int)s[i] - 96];
  int ans = 0;
  for (int i = 1; i <= 26; i++) ans += a[i];
  if (!ans) ans = 1;
  printf("%d\n", ans);
}
