How do I use enourmous integers?

I've made a program that uses iostream.h and am trying to calculate Einsteins famous equation (E=m*(c*c)) but the program has problems with such large integers. I know that I probably should use a different (or one more) header file, but witch?
I use Borland c++ compiler so I already have quite a lot of header files (no need for me to download (probably)).

Code:

int relativity()                              
 {                                            
 int e;                                       
 int mj;
 float m;                                     
 int speedoflightinsquare;
speedoflightinsquare=90000000000000000;
 cout<<"\nDefine the mass in Kg: ";           
 cin>>m;
 e=m*speedoflightinsquare;
 mj=e/1000000;
 cout<<"The energy in the object is approximately: "<<mj;
 return 0;
 }
 //mj=MegaJoule

The next size up from an int is a long. If you don't want negative numbers, make it an unsigned long to give you a higher range. My Borland 5.5 compiler has an unsigned long as 4294967295UL.
It also has one of these:
_UI64_MAX 18446744073709551615ui64
but I've never used anything that size!

Check your limits.h file (found in your compilers include directory). That should give you a clue as to what you can use. Remember it might not be portable though.

The other thing to do is scale down the numbers

Thanx, i'll try that soon!