Fortigate 81e - Public nat problem for PPTP

[Contract]

Hi,

I have pool NAT and i use in policy ipv4 all trafic form interface "X" use this NAT.

On interface "X" is connected another router - R2

I want nat Public IPv4 on this router address R2 but only for vpn PPTP (GRE protocol port 1723) 

All traffic from R2 use NAT pool

But if i want connected from Internet to R2 I use Public IP .

It's only working when i dont use Nat pool and all packcets use Public IP.

Pls help - what i'm must do in order to all traffic use Nat pool but Public IP use only vpn PPTP.