#include <bits/stdc++.h>
using namespace std;
int main(void) {
  cin.sync_with_stdio(false);
  string S;
  cin >> S;
  map<char, int> count;
  for (auto ch : S) {
    ++count[ch];
  }
  int odd = 0;
  int even = 0;
  for (auto kv : count) {
    if (kv.second % 2) {
      ++odd;
    } else {
      ++even;
    }
  }
  if (odd == 0) {
    cout << 1 << endl;
  } else {
    cout << odd << endl;
  }
  return 0;
}
