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

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    string s; cin >> s;
    int N = s.size(),answer = 0;
    for(int i=0; i<N/2; i++){
        if(s.at(i) != s.at(N-1-i)) answer++;
    }
    cout << answer << endl;
}