#include <stdio.h>
#define MAX_LINE_N 15

int main()
{
	double point[MAX_LINE_N+1][2]={0,0};
	int i=0, j=0;
	double area=0.0;
	//get x and y from stdin
	while(scanf("%lf%lf", &point[i][0], &point[i][1]) != EOF)
		i++;
	//copy the first point to the position behind the last point
	point[i][0] = point[0][0];
	point[i][1] = point[0][1];
	//compute the area of polygon by dividing it into several trapezoids
	for (j=0; j<i; j++)
		area += (point[j][1] - point[j+1][1]) * (point[j][0] + point[j+1][0]) / 2;
	//print the area retaining two decimal
	printf("%.2f", area);
	return 0;
}