#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rfor(i, m, n) for (int i = (m); i >= (n); --i)
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define mp make_pair
#define pb push_back
#define eb emplace_back

using namespace std;
using LL = long long;
using VB = vector<bool>;
using VI = vector<int>;
using VL = vector<LL>;
using VS = vector<string>;
using VD = vector<double>;
using VVI = vector<VI>;
using VVL = vector<VL>;
using PII = pair<int, int>;
using PLL = pair<LL, LL>;
using VP = vector<PII>;
using VPL = vector<PLL>;
const int inf = (int)1e9;
const LL inf_ll = (LL)1e18, MOD = 1000000007;
const double PI = acos(-1.0), EPS = 1e-12;

template <class T>
inline void Sort(T& a) noexcept {
  sort(all(a));
}
template <class T>
inline void RSort(T& a) noexcept {
  sort(rall(a));
}
template <class T>
inline void Reverse(T& a) noexcept {
  reverse(all(a));
}
template <class T>
inline void Unique(T& a) noexcept {
  a.erase(unique(all(a)), a.end());
}
template <class T>
inline T Sorted(T a) noexcept {
  Sort(a);
  return a;
}
template <class T>
inline T RSorted(T a) noexcept {
  RSort(a);
  return a;
}
template <class T>
inline T Reversed(T a) noexcept {
  Reverse(a);
  return a;
}
template <class T>
inline T Uniqued(T a) noexcept {
  Unique(a);
  return a;
}
template <class T>
inline auto Max(const T& a) noexcept {
  return *max_element(all(a));
}
template <class T>
inline auto Min(const T& a) noexcept {
  return *min_element(all(a));
}
template <class T>
inline int MaxPos(const T& a) noexcept {
  return max_element(all(a)) - a.begin();
}
template <class T>
inline int MinPos(const T& a) noexcept {
  return min_element(all(a)) - a.begin();
}
template <class T, class U>
inline int Count(const T& a, const U& v) noexcept {
  return count(all(a), v);
}
template <class T, class U>
inline int Find(const T& a, const U& v) noexcept {
  auto pos = find(all(a), v);
  return pos == a.end() ? -1 : pos - a.begin();
}
template <class T, class U>
inline U Sum(const T& a, const U& v) noexcept {
  return accumulate(all(a), v);
}
template <class T, class U>
inline int Lower(const T& a, const U& v) noexcept {
  return lower_bound(all(a), v) - a.begin();
}
template <class T, class U>
inline int Upper(const T& a, const U& v) noexcept {
  return upper_bound(all(a), v) - a.begin();
}
template <class T, class P>
inline void RemoveIf(T& a, P f) noexcept {
  a.erase(remove_if(all(a), f), a.end());
}
template <class T>
inline T Age(T n, T m) noexcept {
  return (n + m - 1) / m;
}
template <class T>
inline T Age2(T n, T m) noexcept {
  return Age(n, m) * m;
}
template <class T>
inline T Tri(T n) noexcept {
  return (n & 1) ? (n + 1) / 2 * n : n / 2 * (n + 1);
}
template <class T>
inline T Gcd(T n, T m) noexcept {
  return m ? Gcd(m, n % m) : n;
}
template <class T>
inline T Lcm(T n, T m) noexcept {
  return n / Gcd(n, m) * m;
}
template <class T>
inline T Pow(T a, T n) noexcept {
  T r = 1;
  while (n > 0) {
    if (n & 1) r *= a;
    a *= a;
    n /= 2;
  }
  return r;
}
template <class T>
inline T Powmod(T a, T n, T m = MOD) noexcept {
  T r = 1;
  while (n > 0) {
    if (n & 1)
      r = r * a % m, n--;
    else
      a = a * a % m, n /= 2;
  }
  return r;
}
template <class T>
inline bool chmax(T& a, const T& b) noexcept {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, const T& b) noexcept {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
inline string operator*(string s, int n) noexcept {
  string ret;
  rep(i, n) ret += s;
  return ret;
}

// --- input --- //
#if defined(WIN32)
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#endif
inline int gc() noexcept { return getchar_unlocked(); }
template <class T>
inline void InputF(T& v) noexcept {
  cin >> v;
}
inline void InputF(char& v) noexcept {
  while (isspace(v = gc()))
    ;
}
inline void InputF(string& v) noexcept {
  char c;
  for (InputF(c); !isspace(c); c = gc()) v += c;
}
inline void InputF(int& v) noexcept {
  bool neg = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c); c = gc()) v = v * 10 + (c - '0');
  if (neg) v = -v;
}
inline void InputF(long long& v) noexcept {
  bool neg = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c); c = gc()) v = v * 10 + (c - '0');
  if (neg) v = -v;
}
inline void InputF(double& v) noexcept {
  double dp = 1;
  bool neg = false, adp = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c) || c == '.'; c = gc()) {
    if (c == '.')
      adp = true;
    else if (adp)
      v += (c - '0') * (dp *= 0.1);
    else
      v = v * 10 + (c - '0');
  }
  if (neg) v = -v;
}
template <class T, class U>
inline void InputF(pair<T, U>& v) noexcept {
  InputF(v.first);
  InputF(v.second);
}
template <class T>
inline void InputF(vector<T>& v) noexcept {
  for (auto& e : v) InputF(e);
}
struct InputV {
  int n, m;
  InputV(int N) : n(N), m(0) {}
  InputV(pair<int, int> N) : n(N.first), m(N.second) {}
  template <class T>
  operator vector<T>() noexcept {
    vector<T> v(n);
    InputF(v);
    return v;
  }
  template <class T>
  operator vector<vector<T>>() noexcept {
    vector<vector<T>> v(n, vector<T>(m));
    InputF(v);
    return v;
  }
};
struct Input {
  template <class T>
  operator T() noexcept {
    T v;
    InputF(v);
    return v;
  }
  int operator--(int) {
    int v;
    InputF(v);
    v--;
    return v;
  }
  InputV operator[](int n) noexcept { return InputV(n); }
  InputV operator[](pair<int, int> n) noexcept { return InputV(n); }
  template <class T, size_t W>
  array<vector<T>, W> get(int H) {
    array<vector<T>, W> ret;
    rep(i, H) rep(j, W) {
      T x = *this;
      ret[j].push_back(x);
    }
    return ret;
  }
} in;

// --- output --- //
struct BoolStr {
  const char *t, *f;
  BoolStr(const char* _t, const char* _f) : t(_t), f(_f) {}
} Yes("Yes", "No"), yes("yes", "no"), YES("YES", "NO"), Int("1", "0");
struct DivStr {
  const char *d, *l;
  DivStr(const char* _d, const char* _l) : d(_d), l(_l) {}
} spc(" ", "\n"), no_spc("", "\n"), end_line("\n", "\n"), comma(",", "\n"),
    no_endl(" ", "");
class Output {
  BoolStr B{Yes};
  DivStr D{spc};
  bool isPrint = true;
  void p(double v) { printf("%.20f", v); }
  void p(long double v) { printf("%.20Lf", v); }
  void p(int v) { printf("%d", v); }
  void p(LL v) { printf("%lld", v); }
  void p(char v) { putchar(v); }
  void p(bool v) { printf(v ? B.t : B.f); }
  template <class T>
  void p(const T& v) {
    cout << v;
  }
  template <class T, class U>
  void p(const pair<T, U>& v) {
    p(v.first);
    printf(D.d);
    p(v.second);
  }
  template <class T>
  void p(const vector<T>& v) {
    rep(i, sz(v)) {
      if (i) printf(D.d);
      p(v[i]);
    }
  }
  template <class T>
  void p(const vector<vector<T>>& v) {
    rep(i, sz(v)) {
      if (i) printf(D.l);
      p(v[i]);
    }
  }
  void p(const BoolStr& v) {
    B = v;
    isPrint = false;
  }
  void p(const DivStr& v) {
    D = v;
    isPrint = false;
  }

 public:
  void operator()() { printf(D.l); }
  template <class H>
  void operator()(H&& h) {
    p(h);
    if (isPrint) printf(D.l);
    isPrint = true;
    B = Yes;
    D = spc;
  }
  template <class H, class... T>
  void operator()(H&& h, T&&... t) {
    p(h);
    if (isPrint) printf(D.d);
    isPrint = true;
    operator()(forward<T>(t)...);
  }
  template <class... T>
  void exit(T&&... t) {
    operator()(forward<T>(t)...);
    std::exit(EXIT_SUCCESS);
  }
} out;

int main() {
  string s = in;
  int n = sz(s);
  int ans = 0;
  rep(i, n / 2) { ans += s[i] != s[n - i - 1]; }
  out(ans);
}

Measured memory_bytes: 256