If my DHCP server exist in different network and we used NATTING, how user can get ip from DHCP server?

Okey, 
first : For Natting we need INSIDE LOCAL IP Which will be mapped to INSIDE GLOBAL.

But Since the Network for which you are using a DHCP will have no IP by default.

HOW WILL YOU RUN NATTING AT THE FIRST PLACE>

INSIDE LOCAL                                                                           INSIDE GLOBAL

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IP ADDRESS OF HOSTS WITHIN THE NETWORK                  IP ADDRESS TO WHICH YOU ARE MAPPING THE INSIDE LOCAL.

What im trying to tell you is that your scenario does not make much sense.

As R1 will recive the DHCP discover packet it will convert broadcast packet into unicast(as it is a relay agent) and DHCP discover will be NATTED and forwarded to DHCP server and using same process it will get IP address.

DHCP server should have a route for PRIVATE NETWORK.