#include <iostream>
using namespace std;


int main() {
	string input_s;
	int input_n;
	cin >> input_n;
	cin >> input_s;

	int size = input_n * 2;
	char* arr = new char[size];
	bool flag = true;
	int index;
	int cnt = 0;

	for (int a = 0; a < input_n; a++) {
		arr[a] = input_s[a];
		arr[a + input_n] = input_s[a];

		if (input_s[a] == input_s[input_n - 1] && flag == true) {
			index = a;
			flag = false;
		}
	}



	for (int a = index; a < index + input_n; a++) {
		if (arr[a] == input_s[input_n + index - a - 1]) {
			cnt++;
		}
	}

	cout << ((cnt == input_n) ? index : -1);
	return 0;
}