#include<fstream>
#include<cstdio>
#include<cmath>
using namespace std;
int n;
struct Punct{double x,y;};
Punct P[100100];
double arie;

inline double Det(Punct A,Punct B)
{
	return A.x*B.y-A.y*B.x;
}

int main()
{
	int i;
	freopen("aria.in","r",stdin);
	scanf("%d",&n);
	for(i=1;i<=n;i++)
		scanf("%lf %lf",&P[i].x,&P[i].y);
	P[0]=P[n];
	for(i=0;i<n;i++)
		arie+=Det(P[i],P[i+1]);
	arie=abs(arie);
	arie/=2.0;
	freopen("aria.out","w",stdout);
	printf("%.6lf\n",arie);
	return 0;
}