#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;

typedef struct Point{
	double x,y;
};

double areaTriangle(Point a,Point b){
	return (a.x*b.y - a.y*b.x)/2;
}

int main(){
	int n;
	Point *ver;
	double area=0;
	
	cin>>n;
	ver = new Point[n];
	
	for(int i=0;i<n;++i){
		cin>>ver[i].x>>ver[i].y;
	}
	
	for(int i=0;i<n;++i){
		area += areaTriangle(ver[i],ver[(i+1)%n]);
	}
	
	cout << fixed << setprecision(1) << area << endl;
}