#include<cstdio>
using namespace std;
FILE*f=fopen("aria.in","r");
FILE*g=fopen("aria.out","w");
int n;
double aria;
struct puncte
{
	double x,y;
}v[100001];
int main()
{
	fscanf(f,"%d",&n);
	for(int i=1;i<=n;++i)
		fscanf(f,"%lf%lf",&v[i].x,&v[i].y);
		
	for(int i=1;i<n;++i)
		aria+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
	
	aria+=(v[n].x*v[1].y-v[1].x*v[n].y)/2;
	
	if(aria<0)
	aria=-aria;
	fprintf(g,"%lf",aria);
	
	fclose(f);
	fclose(g);
	return 0;
}