import sys

N = int(input())
A = list(map(int,input().split()))
ans = 0
while A and A[-1] == A[0]:
    A.pop()
if not A:
    print(0)
    exit()
ans = 1
s =set()
left = 0
while left < len(A):
    if A[left] not in s:
        s.add(A[left])
        right = left
        while right < len(A) and A[right] == A[left]:
            right += 1
        left = right
    else:
        print(-1)
        exit()