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

using namespace std;

struct Point{
	double x,y;
};


int main(){

	int n;
	scanf("%d",&n);
	double gaiseki,sum = 0;

	Point points[n];

	for(int i=0; i < n; i++){
		scanf("%lf %lf",&points[i].x,&points[i].y);
	}
	sum = points[n-1].x*points[0].y - points[n-1].y*points[0].x;
	for(int i=0; i < n-1; i++){
		gaiseki = points[i].x*points[i+1].y - points[i].y*points[i+1].x;
		sum += gaiseki;
	}

	printf("%.1lf\n",fabs(sum)/2);

    return 0;
}