#include <bits/stdc++.h>
using ll = long signed long;
#define REP(i, n) for (ll i = 0, i##_len = (n); i < i##_len; i++)
using namespace std;

int main() {

    string s; cin >> s;
    vector<int> a('z' - 'a' + 1, 0);
    for (char c : s) a[c - 'a']++;
    ll ans = 0;
    for (int x : a) ans += x % 2;
    ans -= ans/2;
    ans -= s.size() % 2 != 0;
    cout << ans << endl;

}
