//#define NDEBUG
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <utility>
#include <vector>

namespace n91 {

  using i8 = std::int_fast8_t;
  using i32 = std::int_fast32_t;
  using i64 = std::int_fast64_t;
  using u8 = std::uint_fast8_t;
  using u32 = std::uint_fast32_t;
  using u64 = std::uint_fast64_t;
  using isize = std::ptrdiff_t;
  using usize = std::size_t;

  struct rep {
    struct itr {
      usize i;
      constexpr itr(const usize i) noexcept : i(i) {}
      void operator++() noexcept { ++i; }
      constexpr usize operator*() const noexcept { return i; }
      constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
    };
    const itr f, l;
    constexpr rep(const usize f, const usize l) noexcept
      : f(std::min(f, l)), l(l) {}
    constexpr auto begin() const noexcept { return f; }
    constexpr auto end() const noexcept { return l; }
  };
  struct revrep {
    struct itr {
      usize i;
      constexpr itr(const usize i) noexcept : i(i) {}
      void operator++() noexcept { --i; }
      constexpr usize operator*() const noexcept { return i; }
      constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
    };
    const itr f, l;
    constexpr revrep(const usize f, const usize l) noexcept
      : f(l - 1), l(std::min(f, l) - 1) {}
    constexpr auto begin() const noexcept { return f; }
    constexpr auto end() const noexcept { return l; }
  };
  template <class T> auto md_vec(const usize n, const T& value) {
    return std::vector<T>(n, value);
  }
  template <class... Args> auto md_vec(const usize n, Args... args) {
    return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
  }
  template <class T> constexpr T difference(const T& a, const T& b) noexcept {
    if (a < b) {
      return b - a;
    }
    else {
      return a - b;
    }
  }
  template <class T> void chmin(T& a, const T& b) noexcept {
    if (b < a) {
      a = b;
    }
  }
  template <class T> void chmax(T& a, const T& b) noexcept {
    if (a < b) {
      a = b;
    }
  }
  template <class F> class fix_point : private F {
  public:
    explicit constexpr fix_point(F&& f) : F(std::forward<F>(f)) {}

    template <class... Args>
    constexpr decltype(auto) operator()(Args&&... args) const {
      return F::operator()(*this, std::forward<Args>(args)...);
    }
  };
  template <class F> constexpr decltype(auto) make_fix(F&& f) {
    return fix_point<F>(std::forward<F>(f));
  }
  template <class T> T scan() {
    T ret;
    std::cin >> ret;
    return ret;
  }

} // namespace n91
#include <cstdint>

namespace n91 {

  constexpr std::uint_fast64_t totient(std::uint_fast64_t x) noexcept {
    using u64 = std::uint_fast64_t;
    u64 ret = x;
    for (u64 i = static_cast<u64>(2); i* i <= x; ++i) {
      if (x % i == static_cast<u64>(0)) {
        ret -= ret / i;
        x /= i;
        while (x % i == static_cast<u64>(0)) {
          x /= i;
        }
      }
    }
    if (x != static_cast<u64>(1)) {
      ret -= ret / x;
    }
    return ret;
  }

  template <std::uint_fast64_t Modulus,
    std::uint_fast64_t InverseExp =
    totient(Modulus) - static_cast<std::uint_fast64_t>(1)>
    class modint {
    using u64 = std::uint_fast64_t;

    static_assert(Modulus < static_cast<u64>(1) << static_cast<u64>(32),
      "Modulus must be less than 2**32");

    u64 a;

    constexpr modint& negate() noexcept {
      if (a != static_cast<u64>(0)) {
        a = Modulus - a;
      }
      return *this;
    }

    public:
      constexpr modint(const u64 x = static_cast<u64>(0)) noexcept
        : a(x% Modulus) {}
      constexpr u64& value() noexcept { return a; }
      constexpr const u64& value() const noexcept { return a; }
      constexpr modint operator+() const noexcept { return modint(*this); }
      constexpr modint operator-() const noexcept { return modint(*this).negate(); }
      constexpr modint operator+(const modint rhs) const noexcept {
        return modint(*this) += rhs;
      }
      constexpr modint operator-(const modint rhs) const noexcept {
        return modint(*this) -= rhs;
      }
      constexpr modint operator*(const modint rhs) const noexcept {
        return modint(*this) *= rhs;
      }
      constexpr modint operator/(const modint rhs) const noexcept {
        return modint(*this) /= rhs;
      }
      constexpr modint& operator+=(const modint rhs) noexcept {
        a += rhs.a;
        if (a >= Modulus) {
          a -= Modulus;
        }
        return *this;
      }
      constexpr modint& operator-=(const modint rhs) noexcept {
        if (a < rhs.a) {
          a += Modulus;
        }
        a -= rhs.a;
        return *this;
      }
      constexpr modint& operator*=(const modint rhs) noexcept {
        a = a * rhs.a % Modulus;
        return *this;
      }
      constexpr modint& operator/=(modint rhs) noexcept {
        u64 exp = InverseExp;
        while (exp) {
          if (exp % static_cast<u64>(2) != static_cast<u64>(0)) {
            *this *= rhs;
          }
          rhs *= rhs;
          exp /= static_cast<u64>(2);
        }
        return *this;
      }
      constexpr bool operator==(const modint rhs) const noexcept {
        return a == rhs.a;
      }
      constexpr bool operator!=(const modint rhs) const noexcept {
        return a != rhs.a;
      }
  };

  template <class T, std::uint_fast64_t v> class modint_constant {
  public:
    static constexpr T value = static_cast<T>(v);

    using value_type = T;
  };
  template <class T, std::uint_fast64_t v>
  constexpr T modint_constant<T, v>::value;

} // namespace n91

#include <algorithm>
#include <bitset>
#include <iostream>
#include <string>
#include <utility>
#include <vector>

namespace n91 {

  void main_() {
    /*
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    //*/
    const std::string s = scan<std::string>();
    usize ans = 0;
    for (const usize i : rep(0, s.size() / 2)) {
      if (s[i] != s[s.size() - i - 1]) {
        ans += 1;
      }
    }
    std::cout << ans << std::endl;
  }

} // namespace n91

int main() {
  n91::main_();
  return 0;
}


Measured memory_bytes: 256