#include <fstream>
#define x first
#define y second
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n,i;
double ans;
pair<double,double> v[100010];
int main() {
	fin>>n;
	for (i=1;i<=n;i++)
		fin>>v[i].x>>v[i].y;
	v[n+1]=v[1];
	for (i=1;i<=n;i++)
		ans+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
	if (ans==int(ans))
		fout<<int(ans);
	else
		fout<<fixed<<setprecision(6)<<ans;
	return 0;
}