#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int dp[s.size() + 1];
  dp[0] = 0;
  int sum[26] = {0};
  for (int i = 0; i < s.size(); i++) {
    sum[s[i] - 'a']++;
    if (sum[s[i] - 'a'] % 2 == 1)
      dp[i + 1] = dp[i] + 1;
    else
      dp[i + 1] = dp[i] - 1;
  }
  cout << max(1, dp[s.size()]) << endl;
  return 0;
}
