#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <queue>
#include <vector>
#include <utility>
#define EPS 1e-10
using namespace std;
typedef long long ll;
struct Point{
	double x,y;
	Point(double x = 0.0,double y = 0.0): 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 lambda){
		return Point(x * lambda,y * lambda);
	}
	Point operator / (double lambda){
		return Point(x / lambda,y / lambda);
	}
	double norm(){
		return x * x + y * y;
	}
	double abs_(){
		return sqrt(norm());
	}
	bool operator == (const Point &p)const
	{
		return abs(x - p.x) < EPS && abs(y - p.y) < EPS;
	}
	bool operator < (const Point &p)const
	{
		if(abs(x - p.x) < EPS) return y - p.y < -EPS;
		else return x - p.x < EPS;
	}
};
typedef Point Vector;
struct Segment{
	Point p1,p2;
	Segment(Point p1 = Point(),Point p2 = Point()): p1(p1),p2(p2) {}
};
typedef Segment Line;
struct Circle{
	Point c;
	double r;
	Circle(Point c = Point(),double r = 0.0): c(c),r(r) {}
};
typedef vector<Point> Polygon;
double dot(Vector a,Vector b){
	return a.x * b.x + a.y * b.y;
}
double det(Vector a,Vector b){
	return a.x * b.y - b.x * a.y;
}
int n;
double area(Polygon p){
	double res = 0.0;
	for(int i=1;i<n-1;i++){
		res += det(p[i] - p[0],p[i+1] - p[0]) / 2.0;
	}
	return res;
}
Polygon p;
int main(){
	scanf("%d",&n);
	for(int i=1;i<=n;i++){
		Point p0;
		scanf("%lf%lf",&p0.x,&p0.y);
		p.push_back(p0);
	}
	printf("%.1f\n",area(p));
	return 0;
}

