#include <bits/stdc++.h>

using namespace std;
using i64 = long long;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	string s;
	cin >> s;
	if (s.length() & 1) {
		s.erase(s.begin() + s.length() / 2, s.begin() + s.length() / 2 + 1);
	}
	
	int i = 0, j = s.length() - 1;
	int ans = 0;
	while (i < j) {
		if (s[i] != s[j]) {
			ans++;
			if (s[i + 1] != s[j - 1]) {
				s[i + 1] = s[j - 1] = 'a';
			} else if (i + 2 < j) {
				s[i + 1] = s[j - 1] = 'a';
				s[i + 2] = s[j - 2] = 'a';   
				ans++;
			}
			s[i] = s[j] = 'a';
			i += 2, j -= 2;
		} else {
			i++, j--;
		}
	}

	cout << ans << '\n';

	return 0;
}