#include<cstdio>
#include<utility>
#define NMAX 100005
#define x first
#define y second
using namespace std;
FILE *fin,*fout;
int n,i;
pair<double,double> v[NMAX];
void read()
{
	fin=fopen("aria.in","r");
	fscanf(fin,"%d",&n);
	for(i=0;i<n;i++)
		fscanf(fin,"%lf %lf",&v[i].x,&v[i].y);
	v[n++]=v[0];
	fclose(fin);
}
int main()
{
	read();
	double aria=0;
	for(i=0;i<n;i++)
		aria+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
	aria/=2;
	fout=fopen("aria.out","w");
	fprintf(fout,"%lf\n",(aria>=0)?aria:-aria);
	fclose(fout);
	return 0;
}