def RunLengthEncoding(S):
    res = []
    N = len(S)
    i = 0
    while i < N:
        cnt = 1
        while i < N - 1 and S[i] == S[i + 1]:
            cnt += 1
            i += 1
        res.append((S[i], cnt))
        i += 1
    return res


def main():
    N = int(input())
    A = list(map(int, input().split()))
    rle = RunLengthEncoding(A)

    if len(rle) == 1:
        return 0

    ans = 0
    if rle[0][0] == rle[-1][0]:
        rle.pop()
        ans = 1

    memo = set()
    for color, _ in rle:
        if color in memo:
            return -1
        memo.add(color)
    return ans


print(main())