N = int(input())

A = list(map(int,input().split()))

dic = {}

import sys
for i in range(N):

    if A[i] != A[i-1]:

        if A[i] in dic:
            print (-1)
            sys.exit()

        dic[A[i]] = 1

if len(dic) <= 1:
    print (0)
elif A[0] == A[-1]:
    print (1)
else:
    print (0)