#include<stdio.h>
#include<math.h>

#include<iostream>
#include<fstream>

#include <bits/stdc++.h>

using namespace std;

int main(){
	
	//freopen("aria.in", "r", stdin);
	//freopen("aria_test8.in", "r", stdin);
	//freopen("aria.out", "w", stdout);

	ifstream fin("aria.in");
	//ifstream fin("aria_test8.in");
	ofstream fout("aria.out");

	int N;
	//scanf("%d",&N);
	fin>>N;

	long double x0, y0, x, y, x1, y1; 
	//scanf("%Lf %Lf", &x, &y);
	fin>>x>>y;

	x0=x; y0=y;

	long double A=0;
	for(int i=1;i<N;i++){
		//scanf("%Lf %Lf", &x1, &y1);
		fin>>x1>>y1;
		A+=(x*y1-x1*y);
		x=x1; y=y1;
	}

	A+=(x*y0-x0*y);
	A/=2.0f;

	//printf("%.6f\n",fabs(A));

	fout<<fixed<<setprecision(6)<<fabs(A)<<endl;
	
	fout<<fabs(A)<<endl;
 
    fin.close();
    fout.close();

	return 0;
}