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

int main() {
    int cnt[26]{}, ans = 0;
    string s;
    cin >> s;
    for(auto c : s){
        cnt[c-'a']++;
    }
    REP(i,26){
        ans += cnt[i]%2;
    }
    cout << ans/2 << endl;
    return 0;
}
