#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
bool cmin(A &a, const B &b) {
  return a > b ? (a = b, true) : false;
}
template <typename A, typename B>
bool cmax(A &a, const B &b) {
  return a < b ? (a = b, true) : false;
}
const double PI = acos(-1);
const double EPS = 1e-7;
int inf = sizeof(int) == sizeof(long long) ? 9e18 : 1e9;
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};
int main() {
  string s;
  cin >> s;
  int out = 0;
  for (int i = 0; i < 26; i++) {
    int co = 0;
    for (int j = 0; j < s.size(); j++)
      if (s[j] - 'a' == i) co++;
    if (co % 2) out++;
  }
  if (!out) out++;
  cout << out << endl;
}
