#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct vec2{
	double x, y;
};

int n;
vector<vec2> poly;

double det(const vec2 &a, const vec2 &b, const vec2 &c){
	return (a.x-c.x)*(b.y-c.y) - (b.x-c.x)*(a.y-c.y);
}

double yote(int a){
	return det(poly[a], poly[(a+1)%n], {0,0});
}

int main(){
	fin >> n;
	for(int i = 0; i < n; ++i){
		vec2 a;
		fin >> a.x >> a.y;
		poly.push_back(a);
	}
	
	double area = 0;
	for(int i = 0; i < n; ++i){
		area += yote(i);
	}
	fout << area/2;
	return 0;
}