#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9, MOD = 1e9 + 7, around[] = {0, 1, 1, -1, 0, -1, 1, 0, 0};
const long double PI = abs(acos(-1));
int c = 0, li[30] = {};
string s;
int main() {
  cin >> s;
  for (int i = (0); i < (s.size()); i++) li[s[i] - 'a']++;
  for (int i = (0); i < (30); i++)
    if (li[i] % 2) c++;
  cout << (c ? c : 1) << endl;
  return 0;
}
