N = int(input())
A = [int(a) for a in input().split()]

newA = [A[0]]
for i in range(1, N):
    if newA[-1] == A[i]:
        continue
    newA.append(A[i])

N = len(newA)
A = newA
ans = 0
num = [0]*(N+1)
for i in range(N):
    if num[A[i]] == 0:
        num[A[i]] += 1
    elif A[i-1] == A[i]:
        num[A[i]] += 1
    elif i == N-1 and A[0] == A[i] and N > 1:
        ans = 1
    else:
        ans = -1
        break
print(ans)