#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using vll = vector<ll>;
using vvl = vector<vll>;
using pll = pair<ll,ll>;
#define rep(i,N) for(ll i=0;i<N;++i)
#define rrep(i,N) for(ll i=N-1; i>=0; --i)
#define all(v) (v).begin(), (v).end()

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    string S;cin >> S;
    ll N=S.size();
    ll ans=0;
    rep(i,N/2){
        if(S[i]!=S[N-i-1])ans++;
    }
    cout << ans << endl;
    return 0;
}