#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
  string s;
  cin>>s;
  int n=s.size();
  int c[26]={};
  for(int i=0;i<n;i++) c[s[i]-'a']++;
  int x=0;
  for(int i=0;i<26;i++) x+=c[i]%2;
  if(x==0) cout<<0<<endl;
  else{
    if(n%2) cout<<x/2<<endl;
    else cout<<(x+1)/2<<endl;
  }
  return 0;
}