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

int main() {
    string S;
    cin >> S;
    int i = 0;
    int ans = 0;
    while(i <= S.size() - 1 - i){
        if(S[i] != S[S.size() - 1 - i]) ans++;
        i++;
    }
    cout << ans << endl;
    return 0;
}