from collections import *

N = int(input())
A = list(map(int, input().split()))
if N == 2:
    print(0)
    exit()

ans = 0
c = -1
if A[0] == A[-1]:
    c = A[0]
    A = deque(A)
    while A:
        if A[0] == c:
            A.popleft()
        else:
            break
    while A:
        if A[-1] == c:
            A.pop()
        else:
            break
    A = list(A)
    if A == []:
        print(0)
        exit()
    ans = 1

seen = [0] * (N + 1)
pre = -1
for a in A:
    if (a != pre and seen[a]) or a == c:
        print(-1)
        exit()
    pre = a
    seen[a] = 1

print(ans)