#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using ll = long long;
#define rep(i, a, b) for (ll i = a; i < b; ++i)
#define rrep(i, a, b) for (ll i = a; i >= b; --i)

// --- 初期設定（入出力の高速化と小数15桁出力） ---
struct Init {
  Init() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);
  }
} init;
// ------------------------------------------------

int main() {
  string s;
  cin >> s;
  ll front = 0;
  ll back = s.size() - 1;
  ll ans = 0;

  while(front < back){
    if(s[front] != s[back]){
      ++ans;
    }
    ++front;
    --back;
  }

  cout << ans << "\n";

  return 0;
}

// Pre-written library: https://github.com/ssmbc2929bartok/cp-env