# problem 1
N = int(input())
K = list(map(int, input().split()))
d = []
d.append(K[0])
for i in range(1, N):
    if K[i] == d[-1]:
        pass
    else:
        d.append(K[i])
ALL = len(d)

# 施行は行われても一回までである

if d[0] == d[-1]:
    if len(d) == 1:
        print(0)
        exit()
    else:
        del d[0]
        if len(d) == len(set(d)):
            print(1)
        else:
            print(-1)
        exit()

else:
    if ALL == len(set(d)):
        print(0)
    else:
        print(-1)
    exit()