N = int(input())
A = list(map(int, input().split()))

ans = [20, 20, 20]


def f(B, u):
    global ans
    NB = len(B)
    ret = False
    for i in range(NB - 2):
        for j in range(i + 1, NB - 1):
            for k in range(j + 1, NB):
                if (B[i] < B[j] and B[k] < B[j] and B[i] != B[k]) \
                        or (B[i] > B[j] and B[k] > B[j] and B[i] != B[k]):
                    C = B[:i] + B[i + 1:j] + B[j + 1:k]
                    if k + 1 < N:
                        C += B[k + 1:]
                    x = not f(C, u + 1)
                    if u == 0 and x == True:
                        ans = min(ans, [i, j, k])
                    ret |= x
    return ret


f(A, 0)
if ans == [20, 20, 20]:
    print(-1)
else:
    print(ans)