#include <bits/stdc++.h>
using namespace std;

int main(void) {
  string s;cin >> s;
  int n = s.size();  
  int l = 0, r = n - 1;
  int ans = 0;
  while(r >= l) {
    ans += (s[l] != s[r]);
    l++;r--;
  }
  cout << ans << '\n';
  return 0;
}