Global variables little help pls

Hi guys I need help with my code this is my homework "Write a C++ program that will calculate the value of “e” (Euler’s number, e = 2.71828…) using a function you create named “find_e”. It shall have no arguments and return no values using the return statement. All transfer of information to and from the function must be in the form of global variables. The function “find_e” must be contained in a file separate file the file containing the “main” function . All output must be from the main program"

I made the program without global variables so you guys can see how it should look
/*#include <iostream>
#include <cmath>


using namespace std;

int main ()

{
double e;
double n;
double accuracy;
int euler = 2.7182818284590452353602874713527;

cout <<"insert the n value "<<endl;
cin>>n;

e=(1+1/n);
e=pow(e,n);


cout<<e<<endl;

accuracy=abs(euler-e);

cout<<"the accuracy is "<<accuracy<<endl;
system ("pause");

return 0;}
*/


and with global variables

this is my function.cpp

#include <iostream>


extern double n;
extern double e;
extern double accuracy;
double euler =2.7182818284590452353602874713527;


void f()
{

e=(1+1/n);
e=pow(e,n);

accuracy=abs(euler-e);

};

*/

and main.cpp

/*
#include <iostream>
#include <cmath>


using namespace std;

double n;
double e;
double accuracy;


int main ()

{


cout <<"insert the n value "<<endl;
cin>>n;


cout<<::e<<endl;



cout<<"the accuracy is "<<::accuracy<<endl;
system ("pause");

return 0;}

*/

I just get a 0 as answer D: