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

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

int n;
long double x,y,a,b,c,d,arie;

int main() {

	fin>>n;
	fin>>a>>b;
	x=a;
	y=b;
	for(int i=1;i<n;i++)
	{
		fin>>c>>d;
		arie+=a*d-b*c;
		a=c;
		b=d;
	}
	arie+=a*y-b*x;
	if(arie<0)	arie=-arie;
	
	fout<<fixed<<setprecision(6)<<arie/2;
	return 0;
}