#include <cstdio>
#include <algorithm>
#define INF 2140000000
#define MaxN 100001
using namespace std;

long double X[MaxN],Y[MaxN],Area=0;
int N;
int main()
{
    freopen("arie.in","r",stdin);
    freopen("arie.out","w",stdout);
	scanf("%d",&N);
	for(int i=0;i<N;i++)
		scanf("%Lf%Lf",&X[i],&Y[i]);
	X[N]=X[0],Y[N]=Y[0];
	for(int i=0;i<N;i++)
		Area+=X[i]*Y[i+1]-X[i+1]*Y[i];
	printf("%f",fabs(Area/2.0));
	return 0;
}