import java.util.Scanner;

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();

		int[] c = new int[n + 1];
		c[a[0]]++;
		for (int i = 1; i < n - 1; i++) {
			if (a[i] != a[i - 1] && c[a[i]] > 0) {
				for (int j = i + 1; j < n; j++) {
					if (a[i] != a[j]) {
						System.out.println(-1);
						return;
					}
				}
				System.out.println(1);
				return;
			}
			c[a[i]]++;
		}
		if (a[0] == a[n - 1]) {
			System.out.println(1);
		} else {
			System.out.println(0);
		}
	}
}