#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
int main() {
  string S;
  cin >> S;
  array<int, 26> count = {0};
  for (int i = 0; i < (S.size()); ++i) {
    ++(count[S[i] - 'a']);
  }
  int result = 0;
  for (int a = 0; a < (26); ++a) {
    if (count[a] % 2 == 1) {
      ++result;
    }
  }
  cout << max(result, 1) << endl;
}
