Student details using inside the class and outside the class

#include<iostream.h>
#include<conio.h>
class stud                            //Outside Class
{
 public:
   int regno,m1,m2,m3,tot;
   float avg;
   char name[20];
};
void main()                          //Main Function
{
 clrscr();
 stud s;                             //Object For Outside Class
 class studin                        //Inside Class
 {
  public:
   void stumark()
   {
    cout<<"Enter Student Details:";
    cout<<"\n\tReg.No :";
    cin>>s.regno;
    cout<<"\tName:";
    cin>>s.name;
    cout<<"\tMark1:";
    cin>>s.m1;
    cout<<"\tMark1:";
    cin>>s.m2;
    cout<<"\tMark1:";
    cin>>s.m3;
    s.tot=s.m1+s.m2+s.m3;
    s.avg=s.tot/3;
    cout<<"\n\tStudent Details";
    cout<<"\n\t  Number : "<<s.regno;
    cout<<"\n\t   Name  : "<<s.name;
    cout<<"\n\t   Mark1 : "<<s.m1;
    cout<<"\n\t   Mark2 : "<<s.m2;
    cout<<"\n\t   Mark3 : "<<s.m3;
    cout<<"\n\t   Total : "<<s.tot;
    cout<<"\n\t Average : "<<s.avg;
   }
 };
 studin s1;                              //Object For inside Class
 s1.stumark();
 getch();
}