import java.util.Scanner;

public class Main {
	static Scanner scan = new Scanner(System.in);
	static int N;
	static int[] A;
	static boolean[] F;
	
	public static void main(String[] args) {
		N = scan.nextInt();
		A = new int[N];
		F = new boolean[N];
		for(int i=0; i<N; i++) {
			A[i] = scan.nextInt();
		}
		
		if(A[0] == A[N-1]) {
			int now = A[0];
			for(int i=1; i<N; i++) {
				if(now != A[i]) {
					if(check(i)) {
						System.out.println(1);
					} else {
						System.out.println(-1);
					}
					System.exit(0);
				}
			}
			System.out.println(0);
		} else {
			if(check(0)) {
				System.out.println(0);
			} else {
				System.out.println(-1);
			}
		}
	}
	
	static boolean check(int first) {
		int count = 0;
		int last = -1;
		for(int i=first; i<N; i++) {
			if(last == A[i]) {
				count++;
			} else {
				count = 0;
			}
			if(count == 0 && F[A[i]-1]) {
				return false;
			}
			F[A[i]-1] = true;
			last = A[i];
		}
		return true;
	}
}