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

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

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

int main() {
	
	fin>>n;
	n--;
	fin>>a>>b;
	x=a;	y=b;
	while(n--)
	{
		fin>>c>>d;
		arie+=(a*d-b*c);
		a=c;	b=d;
	}
	arie+=(a*y-b*x);
	if(arie<0)	arie*=-1;
	arie/=2;
	cout<<fixed<<setprecision(5)<<arie<<"\n";
}