#include <stdio.h>
#include <stdlib.h>

#define N 7

typedef struct coord_ {
 double x;
 double y;
 } coord;

// рекурсивная структура - кольцо.
typedef struct ring_ {
 coord *data;
 struct ring_ *next;
 struct ring_ *prev;
 } Ring;

//  Делаем элемент и заполняем данными

Ring *makenew( Ring *ring, coord *data )
 {
	ring = (Ring *)malloc(sizeof(Ring));
	ring -> next = ring -> prev = NULL;
	ring -> data = data;
	return ( ring );
 }

// Делаем элемент и связываем с данным всеми способами. Для случая 2-х эл-ов.  
//
//	Было				Станет
//
//       ring 				ring <-> ring->next 

Ring *linkwith( Ring *ring, coord *data )
 {
	ring -> next=(Ring *)malloc(sizeof(Ring));
	ring -> next -> data=data;
	ring -> next -> prev=ring;
	ring -> prev = ring -> next;
        ring -> next -> next = ring;
	return (ring->next);
 }

//
//		Было				Станет
//
//    will_be_before <-> ring	will_be_before <-> _new_ <-> ring 


Ring *placeafter( Ring *will_be_before, coord *data )
 {
	Ring *temp1, *temp2;
	temp1 = will_be_before -> next;
	temp2 = makenew( temp2, data );
        temp2 -> next = temp1;
        temp1 -> prev = temp2;
        temp2 -> prev = will_be_before;
        will_be_before -> next = temp2;
        return (will_be_before->next);
 }

//
//		Было				Станет
//
//     ring <-> will_be_after 		ring <-> _new_ <-> will_be_after 


Ring *placebefore( Ring *will_be_after, coord *data )
 {
	Ring *temp1, *temp2;
	temp1 = will_be_after -> prev;
	temp2 = makenew( temp2, data );
        temp2 -> prev = temp1;
        temp1 -> next = temp2;
        temp2 -> next = will_be_after;
        will_be_after -> prev = temp2;
        return (will_be_after->prev);
 }

//
//	Было				Станет
//
//    1 <-> ring <-> 2 			1 <-> 2 

void remove_elem( Ring *ring )
 {
	ring -> prev -> next = ring -> next;
	ring -> next -> prev = ring -> prev;
	free ( ring );
 }



double sin_angle( Ring *ring1, Ring *ring2, Ring *ring3 )
 {
	double x1,x2,y1,y2;
	x1 = ring1 -> data -> x - ring2 -> data -> x;
	y1 = ring1 -> data -> y - ring2 -> data -> y;
	x2 = ring3 -> data -> x - ring2 -> data -> x;
	y2 = ring3 -> data -> y - ring2 -> data -> y;
	return ( x1 * y2 - y1 * x2 );
 }

double sgn_cos_angle( Ring *ring1, Ring *ring2, Ring *ring3 )
 {
	double x1,x2,y1,y2;
	x1 = ring1 -> data -> x - ring2 -> data -> x;
	y1 = ring1 -> data -> y - ring2 -> data -> y;
	x2 = ring3 -> data -> x - ring2 -> data -> x;
	y2 = ring3 -> data -> y - ring2 -> data -> y;
	return ( x1 * x2 + y1 * y2 );
 }

void convex_it( Ring *last )
 {
	if (sin_angle(last->prev->prev, last->prev, last) == 0 )
		{ 
		 if ( sgn_cos_angle( last->prev->prev, last->prev, last ) <0 )
		  remove_elem(last->prev); 
  		} else

	while ( sin_angle ( last -> prev -> prev, last -> prev, last ) >= 0 )
         remove_elem( last -> prev );

	if (sin_angle(last->next->next, last->next, last) == 0 )
		{ 
                 if ( sgn_cos_angle(last->next->next, last->next, last )<0 )
                  remove_elem(last->next);
                } else

	while ( sin_angle ( last -> next -> next, last -> next, last ) <= 0 )
         remove_elem( last -> next );
 }

void print_ring( Ring *last )
{
	Ring *temp;
	temp=last;
        for(;;)
		{
 	 	 printf("Printing Ring: x:%lf y:%lf\n",last->data->x,last->data->y);
          	 last = last -> next;
		 if( last == temp | last == NULL ) break;
		}
}

void main()
{
 coord *point;
 Ring *ring, *last, *new, *temp;
 int i;
 point=(coord *)malloc(sizeof(coord)*N); 
 for (i=0;i<N;i++) scanf("%lf %lf",&( (point+i)->x ),&( (point+i)->y));
 ring=makenew( ring, point );
 last=linkwith( ring , (point+1) );

 for( i=2; i<N; i++)
  {
	temp = makenew( temp, point+i );
	if ( sin_angle( last->prev, last, temp) >= 0 )
		last = placebefore( last, point+i );
	else last = placeafter ( last, point+i );
     
        convex_it(last);
   }
 print_ring(last);

}