/*
 * @(#) polyarea.c - A free-shape polygon area calculator.
 * (c) 1997 Ivan Maidanski <ivmai@chat.ru> http://ivmai.chat.ru
 * Freeware program source. All rights reserved.
 **
 * Language: ANSI C
 * Tested with: Watcom C 16/32 v10.0
 * Last modified: 1997-11-13 11:40:00 GMT+03:00
 */

/* Input data format: N x1 y1 ... xn yn */

#include <stdio.h>
#include <math.h>

#define MAXN 100

double x[MAXN],y[MAXN];

int input_polygon(double x[], double y[])
{
 int i,n;
 scanf("%d",&n);
 if (n>MAXN)
  return -1;
 for (i=0;i<n;i++)
  scanf("%f%f",&x[i],&y[i]);
 return n;
}

double triangle_area(double x1, double y1, double x2, double y2,
                    double x3, double y3)
{
 return ((x1-x2)*(y3-y2)-(x3-x2)*(y1-y2))/2.0;
}

double noncrossed_polygon_area(double x[], double y[], int n)
{
 int i;
 double sum;
 for (i=2,sum=0.0;i<n;i++)
  sum+=triangle_area(x[0],y[0],x[i-1],y[i-1],x[i],y[i]);
 return fabs(sum);
}

int get_seglines_cross(double *x, double *y,
                       double x1, double y1, double x2, double y2,
                       double x3, double y3, double x4, double y4)
{
 double t,d;
 if (!(d=(x4-x3)*(y2-y1)-(x2-x1)*(y4-y3)))
  return 0;
 if ((t=((x1-x3)*(y2-y1)-(x2-x1)*(y1-y3))/d)<=0.0 || t>=1.0)
  return 0;
 if ((t=((x1-x3)*(y4-y3)-(x4-x3)*(y1-y3))/d)<=0.0 || t>=1.0)
  return 0;
 *x=(x2-x1)*t+x1;
 *y=(y2-y1)*t+y1;
 return 1;
}

void shift_list(double x[], int i, int n)
{
 for (;i<n;n--)
  x[n]=x[n-1];
}

void reverse_list(double x[], int i, int j)
{
 double t;
 for (;i<j;i++,j--)
 {
  t=x[i];
  x[i]=x[j];
  x[j]=t;
 }
}

int insert_cross_points(double x[], double y[], int n)
{
 int i,j;
 double cx,cy;
 for (i=1;i<n-1;i++)
  for (j=i+1;j<n;j++)
   if (get_seglines_cross(&cx,&cy,x[i-1],y[i-1],x[i],y[i],
                          x[j],y[j],x[j+1<n? j+1 : 0],y[j+1<n? j+1 : 0]))
   {
    shift_list(x,i,n);
    shift_list(y,i,n);
    n++;
    j++;
    reverse_list(x,i+1,j);
    reverse_list(y,i+1,j);
    j++;
    shift_list(x,j,n);
    shift_list(y,j,n);
    n++;
    x[i]=x[j]=cx;
    y[i]=y[j]=cy;
   }
 return n;
}

int main()
{
 int n=input_polygon(x,y);
 if (n<0)
 {
  printf("Error!\n");
  return 1;
 }
 n=insert_cross_points(x,y,n);
 printf("Area=%f\n",noncrossed_polygon_area(x,y,n));
 return 0;
}