//
//  main.cpp
//  Aria Poligon
//
//  Created by Darius Buhai on 27/10/2018.
//  Copyright © 2018 Darius Buhai. All rights reserved.
//

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

#define MAXL 100100

using namespace std;

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

int n;
double s;

struct point{
    long double x, y;
};

point a[MAXL];

int main() {
    fin>>n;
    for(int i=0;i<n;i++)
        fin>>a[i].x>>a[i].y;
    a[n] = a[0];
    for(int i=0;i<n;i++)
        s += a[i].x*a[i+1].y - a[i+1].x*a[i].y;
    s = s > 0 ? s : -s;
    s = s/2;
    fout<<fixed<<setprecision(5)<<s;
    return 0;
}