#include <bits/stdc++.h>
using namespace std;
string S;
long long dp[200005];
map<char, long long> hist[200005];
bool check(long long left, long long right) {
  long long num = right - left + 1;
  long long odd = 0;
  long long even = 0;
  for (char ch = 'a'; ch <= 'z'; ch++) {
    long long alnum;
    if (left == 0) {
      alnum = hist[right][ch];
    } else {
      alnum = hist[right][ch] - hist[left - 1][ch];
    }
    if (alnum % 2 == 0) {
      even++;
    } else {
      odd++;
    }
  }
  if (num % 2 == 0) {
    if (odd) {
      return false;
    }
    return true;
  } else {
    if (odd == 1) {
      return true;
    }
    return false;
  }
}
int main() {
  cin >> S;
  for (long long i = 0; i < S.size(); i++) {
    dp[i] = INT_MAX;
    if (i == 0) {
      hist[i][S[i]]++;
    } else {
      hist[i] = hist[i - 1];
      hist[i][S[i]]++;
    }
  }
  long long odd = 0;
  long long even = 0;
  for (char ch = 'a'; ch <= 'z'; ch++) {
    if (hist[S.size() - 1][ch] % 2 == 0) {
      even++;
    } else {
      odd++;
    }
  }
  cout << max((long long)1, odd) << endl;
}
