#include<bits/stdc++.h>
#define int long long
using namespace std;
signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    string s; cin >> s;
    map<char, int> mp;
    for(char c : s) mp[c] ++;
    int ans = 0;
    for(auto x : mp) if(x.second % 2) ans ++;
    cout << max(0LL, ans - 1);
}