#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);
}
I2luY2x1ZGU8Yml0cy9zdGRjKysuaD4KI2RlZmluZSBpbnQgbG9uZyBsb25nCnVzaW5nIG5hbWVzcGFjZSBzdGQ7CnNpZ25lZCBtYWluKCkgewogICAgaW9zOjpzeW5jX3dpdGhfc3RkaW8oMCk7CiAgICBjaW4udGllKDApOyBjb3V0LnRpZSgwKTsKICAgIHN0cmluZyBzOyBjaW4gPj4gczsKICAgIG1hcDxjaGFyLCBpbnQ+IG1wOwogICAgZm9yKGNoYXIgYyA6IHMpIG1wW2NdICsrOwogICAgaW50IGFucyA9IDA7CiAgICBmb3IoYXV0byB4IDogbXApIGlmKHguc2Vjb25kICUgMikgYW5zICsrOwogICAgY291dCA8PCBtYXgoMExMLCBhbnMgLSAxKTsKfQ==