#include<iostream>
#include<string>

using namespace std;

int main(){
  string S;
  cin >> S;
  int ans=0;
  int Slen=S.size();
  for(int i=0;i<Slen/2;i++){
    if(S[i]!=S[Slen-1-i]){
      ans++;
    }
  }
  cout << ans << "\n";
  
  return 0;
}