Разбиение программы на файлы С++

#include <iostream>
#include <windows.h>
#include <cstring>
#define _USE_MATH_DEFINES
#include <math.h>
using namespace std;
 
extern double p,n,r,a;
void count();
 
void main()
{
    char s[5];
    n=5;
    r=20;
    count();
    cout<<"\nFor radius=20 and sides count=5 perimeter of a polygon equals: "<<p;
    cout<<endl;
    do 
    {
        cout<<"\nInput the number sides of the polygon: ";
        cin>>n;
        cout<<endl;
        cout<<"Input radius of circle: ";
        cin>>r;
        cout<<endl;
        count();
        cout<<"The perimeter of a polygon equals: " <<p;
        cout<<endl;
        cout<<"\nAgain? (Yes/No): ";
        cin>>s;
    } while (!strcmp(s, "yes") || !strcmp(s, "Yes"));
}

#include <math.h>
#define _USE_MATH_DEFINES
 
void count()
{
    a=2*r*sin(3.14/n);
    p=n*a;
}

#include "file2.h"

#include <File2.h>

#include <File2.cpp>

#include <iostream>
#include <windows.h>
#include <cstring>
#include "File2.h"
 
using namespace std;
 
 void main()
{
        char s[5];
        n=5;
        r=20;
        count();
        cout<<"\nFor radius=20 and sides count=5 perimeter of a polygon equals: "<<p;
        cout<<endl;
        do 
        {
                cout<<"\nInput the number sides of the polygon: ";
                cin>>n;
                cout<<endl;
                cout<<"Input radius of circle: ";
                cin>>r;
                cout<<endl;
                count();
                cout<<"The perimeter of a polygon equals: " <<p;
                cout<<endl;
                cout<<"\nAgain? (Yes/No): ";
                cin>>s;
        } while (!strcmp(s, "yes") || !strcmp(s, "Yes"));
}

#include <math.h>
#define _USE_MATH_DEFINES
double p,n,r,a;
 
void count()
{
        a=2*r*sin(3.14/n);
        p=n*a;
}