import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Scanner;
import java.util.Set;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
		}
		sc.close();

		List<Integer> list = new ArrayList<>();
		list.add(a[0]);
		for (int i = 1; i < n; i++) {
			if (a[i - 1] != a[i]) {
				list.add(a[i]);
			}
		}
		Set<Integer> set = new HashSet<>();
		for (Integer o : list) {
			set.add(o);
		}
		if (set.size() == list.size()) {
			System.out.println(0);
		} else {
			if (set.size() + 1 == list.size() &&
					list.get(0).equals(list.get(list.size() - 1))) {
				System.out.println(1);
			} else {
				System.out.println(-1);
			}
		}
	}
}