Cascade IP3 Calculation

Hi,
I read the following link on IP3 calculation:
http://www.downeastmicrowave.com/PDF/IP3.PDF

There are three stages (stage 1 and stage 3 are amp while stage 2 is bandpass filter) which has the OIP3 and gain respectively:
OIP3 +21dBm(125.9) inf dBm(inf) +25 dBm(316.2)
Gain 15 dB (31.62) -3 dB (.5) 15 dB (31.62)

After calculate cascading OIP3, it gives:
OIP3 = 273.075 = 24.4 dBm , IIP3 = 24.4 - 33 = -8.6 dBm

Where does the 33 come from?
Could you help me on that?
Thanks,

The 33 is due to the fact that they went from OIP3 to IIP3. The difference of IIP to OIP is the Gain.
So if you subtract the Gains from the system and take it away from the OIP3 you will get the IIP.
But it should be 27dB not 33. --> 15+ (-3) +15 = 27