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

int main(){
	long long  n;
	cin >> n;
	if(n >= 4){
		cout << "YES" << endl;
		return 0;
	}
	n++;
	string str,s = " ";
	cin >> str;
	s += str;
	long long i,j;
	for(i = 1; i < n; i++){
		string str1 = "", str2 = "";;
		for(j = i; j + (j + 1 - i) <= n; j++){
			str1 += s[j];
			str2 += s[j+1];
			//			cout << i << " " << j << endl;
			//			cout << "str1 = " << str1 << " str2 = " << str2 << endl;
			if(str1 == str2){
				cout << "YES" << endl;
				return 0;
			}
		}
	}
	cout << "NO" << endl;
	return 0;
}