import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
rle = [A[0]]

for i in range(1, N):
    if A[i]!=rle[-1]:
        rle.append(A[i])

s = set()

for i in range(len(rle)):
    if rle[i] not in s:
        s.add(rle[i])
    else:
        if i!=len(rle)-1:
            print(-1)
            exit()
        elif rle[i]!=rle[0]:
            print(-1)
            exit()

if len(rle)>=2 and rle[0]==rle[-1]:
    print(1)
else:
    print(0)