#include <bits/stdc++.h>

using namespace std;

int main() {
    string s;
    cin >> s;

    if (s.substr(0, s.length() / 2) !=
        s.substr(s.length() / 2)) {
        cout << "YES" << endl;
    } else {
        cout << "NO" << endl;
    }
    return 0;
}