#include<cstdio>
#include<algorithm>
#define point pair<double,double>
#define x first
#define y second
using namespace std;
const int nmax = 100005;
int n,i; double a;
point p[nmax];
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	scanf("%d",&n);
	for(i=1;i<=n;i++) scanf("%lf%lf",&p[i].x,&p[i].y);
	p[n+1]=p[1];
	for(i=1;i<=n;i++) a+=p[i].x*p[i+1].y-p[i].y*p[i+1].x;
	if(a<0) a=-a; a/=2.0;
	printf("%lf\n",a);
	return 0;
}