#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define all(s) (s).begin(), (s).end()
#define rall(s) (s).rbegin(), (s).rend()
#define rep(i,l,r) for(int i=l;i<r;i++)
#define rrep(i,r,l) for(int i=r;i>l;i--)

using ll=long long;
using mint=modint998244353;

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

    string S;cin>>S;
    string T=S;
    reverse(all(T));
    int ans=0;
    int N=S.size();
    rep(i,0,(N-1)/2+1){
        if(S[i]!=T[i])ans++;
    }
    cout<<ans<<endl;
}