def andXorOr(a):
    stack = [a[0], a[1]]
    S = a[0] ^ a[1]
    for i in range(2, len(a)):
        while len(stack) > 0 and stack[-1] >= a[i]:
            S = max(S, stack[-1] ^ a[i])
            stack.pop()
        if len(stack) > 0:
            S = max(S, stack[-1] ^ a[i])
        stack.append(a[i])
    return S