#include <bits/stdc++.h>
using namespace std;
int main() {
string s; cin >> s;
int cnt[30] = {}, ans = 0;
for(size_t i=0; i<s.length(); i++) cnt[ s[i] - 'a' ]++;
for(int i=0; i<26; i++) if(cnt[i] % 2) ans++;
cout << ans / 2 << endl;
return 0;
}
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