#include<fstream>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
typedef struct point{
	double x,y;
};
point a[100001];
int i,n;
double sol=0;
int main()
{
	fin>>n;
	for(i=0;i<n;i++)
	{
		fin>>a[i].x>>a[i].y;
	}
	a[n]=a[0];
	for(i=0;i<n;i++)
	{
		sol+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
	}
	fout<<sol/2;
	
	return 0;
}