#include <fstream>
#include <iostream>
#include <iterator>
#include <algorithm>
//#include <cmath>

#define MAXN 100002

using namespace std;

struct Point
{
    Point() :
        x(0.0),
        y(0.0)
    {}

    Point(long double x_, long double y_) :
        x(x_),
        y(y_)
    {}

    long double x = 0.0;
    long double y = 0.0;
};

istream & operator >> (istream & is, Point & p)
{
    is >> p.x >> p.y;
    return is;
}

ostream & operator << (ostream & os, const Point & p)
{
    os << p.x << " "<< p.y;
    return os;
}

Point points[MAXN];

long double computeTriangleArea(const Point & p1, const Point & p2, const Point & p3)
{
    return .5 * (-p2.x * p1.y + p3.x * p1.y + p1.x * p2.y - p3.x * p2.y - p1.x * p3.y  + p2.x * p3.y);
}

int main()
{
    int n;
    fstream fin("aria.in", fstream::in);
    fstream fout("aria.out", fstream::out);

    fin >> n;
    //cout << n << endl;
    
    copy_n(istream_iterator<Point>(fin), n, points);

    //ostream_iterator<Point> itOut(cout, "\n");
    //copy(points, points + n, itOut);
    
    //long double pivot = -1100000.0;
    
    long double area = points[n-1].x * points[0].y - points[0].x * points[n - 1].y;
    
    for (int i=0; i<n - 1; ++i)
    {
        area += (points[i].x * points[i + 1].y - points[i + 1].x * points[i].y);
    }
    
    area *= .5;
    
    fout << fabs(area) << "\n";

    return 0;
}