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

int main()
{
	string s;
	cin >> s;
	int t = 0;
	for(int i = 0; i < s.length()/2; i++){
		if(s[i] != s[s.length() - i - 1])
			t++;
	}
	cout << t << endl;
 } 