#include <fstream>
#include <iomanip>
using namespace std;

ifstream fi("aria.in");
ofstream fo("aria.out");

struct point{long double x,y;}a[1000000];

long long n,i;

int main(){
	
	fi >> n;
	for (i=1; i<=n; i++) fi >> a[i].x >> a[i].y;
	a[n+1]=a[1];
	long double rs=0;
	for (i=1; i<=n; i++) rs+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
	fo << fixed << setprecision(5) <<  (long double)rs/2;
	return 0;
}