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

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	string s;
	cin >> s;
	int l = 0, r = s.size() - 1, ans = 0;
	while(l < r){
		ans += s[l] != s[r];
		l++;
		r--;
	}
	cout << ans << '\n';
}