#include <fstream>
#include <iostream>
#include <vector>
using namespace std;
struct coord{
	double x, y;
};
vector <coord> v;
int main(){
	int n, i;
	double x, y, r;
	ifstream fin( "aria.in" );
	ofstream fout( "aria.out" );
	fin >> n;
	for( i = 0; i < n; i++ ){
		fin >> x >> y;
		v.push_back( { x, y } );
	}
	r = 0;
	for( i = 0; i < n; i++ ){
		r += ( v[i].x * v[(i + 1) % n].y - v[(i + 1) % n].x * v[i].y ) / 2;
	}
	fout << r;
	return 0;
}