#include <iostream>
#include <algorithm>
#include <string>
#include <stack>
#include <queue>
#include <set>
#include <vector>
#include <map>
#include <cstdio>
#include <cmath>
#include <assert.h>
#define rep(i,a,b) for(int (i)=(a);i<(b);i++)
#define INF 100000000
#define MAX_N 150005
#define EPS 0.000000001
using namespace std;

int main(){
    int cnt=0;
    int a[26]={};
    string s;
    cin>>s;
    rep(i,0,s.size()){
        a[int (s[i]-'a')]++;
    }
    rep(i,0,26){
        if(a[i]%2==1)cnt++;
    }
    cout<<cnt/2<<endl;
    
    return 0;
}