from collections import defaultdict, deque

d = defaultdict(deque)
N = int(input())
A = list(map(int, input().split()))
A += A
l, r = 0, 0
while l < N:
    while r < N+l:
        if (len(d[A[r]])>0 and d[A[r]][-1] == r-1) or len(d[A[r]])==0:
            d[A[r]].append(r)
        else:
            for i in range(l, r):
                d[A[i]].popleft()
            l = r
            if l >= N:
                break
            r -= 1
        r += 1
    if r-l == N:
        if l == 0:
            print(0)
            exit()
        else:
            print(1)
            exit()
print(-1)