#include <cstdio>
#include <algorithm>
#include <queue>
#define INF 2140000000
#define MaXN 1030
using namespace std;

double X1,X2,Y1,Y2,Xstart,Ystart,Area;
int N;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
	scanf("%d",&N);
	scanf("%lf%lf",&Xstart,&Ystart);
	X1=Xstart,Y1=Ystart;
	for(int i=2;i<=N;i++)
	{
		X2=X1,Y2=Y1;
		scanf("%lf%lf",&X1,&Y1);
		Area+=X1*Y2-Y1*X2;
	}
	Area+=Xstart*Y1-Ystart*X1;
	printf("%lf",abs(Area/2.0));
	return 0;
}