#include <stdio.h>
#include <algorithm>
#include <vector>
using namespace std;

int N, A[200200], C[100100];

int main()
{
	scanf ("%d", &N);
	for (int i = 0; i < N; i++){
		scanf ("%d", &A[i]);
		A[i + N] = A[i];
		C[A[i]]++;
	}

	int c = -1;
	for (int i = 1; i <= N; i++) if (C[i]) c++;
	for (int i = 1; i < N; i++) if (A[i] != A[i - 1]) c--;

	if (c == 0){
		puts("0");
		return 0;
	}
	for (int i = N; i < N * 2; i++){
		if (A[i - N] != A[i - N + 1]) c++;
		if (A[i] != A[i - 1]) c--;
		if (c == 0){
			puts("1");
			return 0;
		}
	}
	puts("-1");
	return 0;
}