#include <bits/stdc++.h>
using namespace std;

#define rep(i, a, b) for (ll i = a; i < b; i++)
using ll = long long;

int main(){
    string s;
    cin >> s;
    int n = s.size();
    int l = 0, r = n - 1, ans = 0;
    while(l < r) {
        if(s[l] != s[r]) ans++;
        l++;
        r--;
    }
    cout << ans << endl;
}