#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define REP(i, n) for(int i = 0; i < n; i++)
#define REPR(i, n) for(int i = n; i >= 0; i--)
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define FORR(i, m, n) for(int i = m; i >= n; i--)
#define INF (ll)2e9
#define MOD ((ll)1e9+7)
#define ALL(v) v.begin(), v.end()
#define SZ(x) ((int)(x).size())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define next asdnext
#define prev asdprev
#define bit(n) (1LL<<(n))
#define cauto const auto&
#define println(v) cout << v << "\n";

void show(vector<ll>& arr, int w) {
    REP(i, arr.size()) {
        cout << right << setw(w) << i;
    }
    cout << endl;
    REP(i, arr.size()){
        if (abs(INF - arr[i]) < 1e5) {
            cout << right << setw(w) << (arr[i] == INF ? "INF" : "inf");
        } else {
            cout << right << setw(w) << arr[i];
        }
    }
    cout << endl;
}

void show(vector<vector<ll>>& arr, int w) {
    cout << right << setw(w) << "#";
    REP(i, SZ(arr[0]))  {
        cout << right << setw(w) << i;
    }
    cout << endl;
    REP(i, SZ(arr)) {
        cout << right << setw(w) << i;
        REP(j, SZ(arr[0])) {
            if (abs(INF - arr[i][j]) < 1e5) {
                cout << right << setw(w) << (arr[i][j] == INF ? "INF" : "inf");
            } else {
                cout << right << setw(w) << arr[i][j];
            }
        }
        cout << endl;
    }
}

void show(vector<vector<vector<ll>>>& arr, int w) {
    REP(i, arr.size()) {
        println("i: " + to_string(i));
        show(arr[i], w);
        println("");
    }
}

inline vector<vector<vector<ll>>> make_vector(ll i, ll j, ll k) {
    vector<vector<vector<ll>>> v(i, vector<vector<ll>>(j, vector<ll>(k, 0)));
    return v;
}
inline vector<vector<ll>> make_vector(ll i, ll j) {
    vector<vector<ll>> v(i, vector<ll>(j, 0));
    return v;
}

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template <class InputIterator>
InputIterator adv(InputIterator x, typename std::iterator_traits<InputIterator>::difference_type n) {
    advance(x, n);
    return x;
}

class mod {
    static ll fact[];
public:

    template<class... A>
    static ll mul(A... args) {
        ll res = 1;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = (res * i) % MOD;
        }
        return res;
    }

    static ll power(ll base, ll exp) {
        if (exp == 0) return 1;
        if (exp & 1) {
            return mul(base, power(base, exp - 1));
        } else {
            ll p = power(base, exp / 2);
            return mul(p, p);
        }
    }
    
    static void calc_factorial(int n) {
        fact[0] = 1;
        FOR(i, 1, n+1) {
            fact[i] = mul(fact[i-1], i);
        }
    }

    static ll factorial(int n) {
        if (fact[n] != 0) return fact[n];
        if (n == 0) return 1;
        return fact[n] = mul(n, factorial(n - 1));
    }

    static ll inverse(int n) {
        return power(n, MOD - 2);
    }

    static ll comb(int n, int r) {
        if (r < 0 || r > n) return 0;
        return mul(factorial(n), inverse(factorial(n - r)), inverse(factorial(r)));
    }

    template<class... A>
    static ll div(ll dividend, A... args) {
        ll res = dividend;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = mul(res, inverse(i));
        }
        return res;
    }

    template<class... A>
    static ll add(A... args) {
        ll res = 0;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = (res + i) % MOD;
        }
        return res;
    }

    template<class... A>
    static ll sub(ll l, A... args) {
        ll res = l;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = (res - i + MOD) % MOD;
        }
        return res;
    }
};
ll mod::fact[(int) 2e6 + 1];

int ni() {
    int i;
    cin >> i;
    return i;
}
ll nll() {
    ll l;
    cin >> l;
    return l;
}

int main()
{
    string s;
    cin >> s;

    int N = SZ(s);

    int ans = 0;
    REP(i, N/2) {
        if (s[i] != s[N-1-i]) ans++;
    }

    println(ans);

    return 0;
}

Measured memory_bytes: 256