#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

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