#include <bits/stdc++.h>
using namespace std;
// read from file instead of console:
// freopen("input.txt", "r", stdin);
// freopen("output.txt", "w", stdout);
const int M = 1000000007;
typedef long long ll;
typedef vector<int> vi;
typedef vector<bool> vb;
typedef vector<vector<int>> vvi;
typedef vector<vector<vector<int>>> vvvi;
typedef pair<int, int> pi;
typedef queue<int> qi;
typedef stack<int> si;
typedef priority_queue<int> pqi;
typedef deque<int> dqi;
typedef unordered_set<int> seti;
typedef unordered_map<int,int> mapi;
typedef vector<long long> vll;
typedef vector<vector<long long>> vvll;
typedef vector<vector<vector<long long>>> vvvll;
typedef pair<long long, long long> pll;
typedef queue<long long> qll;
typedef stack<long long> sll;
typedef priority_queue<long long> pqll;
typedef deque<long long> dqll;
typedef unordered_set<long long> setll;
#define loop(i, a, b) for (int i = a; i < b; i++)
#define rloop(i, a, b) for (int i = a; i > b; i--)
#define each(a, c) for (auto a : c)
#define all(x) x.begin(), x.end()
#define PI M_PI
#define PB push_back
#define P push
#define POB pop_back
#define F first
#define S second
#define sz size()
#define R1(a) int a; cin >> a
#define R2(a, b) int a,b; cin >> a >> b
#define R3(a, b, c) int a,b,c; cin >> a >> b >> c
#define R4(a, b, c, d) int a,b,c,d; cin >> a >> b >> c >> d
#define WS(a) cout << a << " "
#define WN(a) cout << a << "\n"
#define WNL cout << "\n"
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
string s;
cin >> s;
int n = s.length();
int r = 0;
int h = n / 2;
loop(i,0,h){
if(s.at(i) != s.at(n-1-i)) r++;
}
cout << r;
return 0;
}
