#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>
#include<map>
#include<string>
#include <math.h>
#include<algorithm>
#include<functional>
#define ll long long
#define inf  999999999
#define pa pair<int,int>
#define EPS (1e-10)
#define equals(a,b) (fabs((a)-(b))<EPS)

using namespace std;

class Point{
	public:
	double x,y;
	Point(double x=0,double y=0):x(x),y(y) {}
	Point operator + (Point p) {return Point(x+p.x,y+p.y);}
	Point operator - (Point p) {return Point(x-p.x,y-p.y);}
	Point operator * (double a) {return Point(x*a,y*a);}
	Point operator / (double a) {return Point(x/a,y/a);}
	double absv() {return sqrt(norm());}
	double norm() {return x*x+y*y;}
	bool operator < (const Point &p) const{
		return x != p.x ? x<p.x: y<p.y;
	}
	bool operator == (const Point &p) const{
		return fabs(x-p.x)<EPS && fabs(y-p.y)<EPS;
	}
};
typedef Point Vector;

struct Segment{
Point p1,p2;
};


double dot(Vector a,Vector b){
	return a.x*b.x+a.y*b.y;
}
double cross(Vector a,Vector b){
	return a.x*b.y-a.y*b.x;
}

//----------------kokomade temple------------

int main(){
int n;
double x,y;
cin>>n;
Point a[110];
for(int i=0;i<n;i++){
cin>>x>>y;
a[i].x=x;
a[i].y=y;
}
a[n]=a[0];
double ans=0.0;
for(int i=0;i<n;i++){
ans=ans+ 0.5*(a[i].x * a[i+1].y -a[i].y *a[i+1].x);
}
printf("%.1f\n",ans);


}