#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

#define NMAX 100001

int n;

struct point{
    double x,y;
};

double area(point a, point b)
{
    return a.x*b.y - a.y*b.x;
}

double total_area;

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

void read()
{
    fin>>n;
    point first,ultim,p;
    fin>>first.x>>first.y;

    ultim = first;

    for(int i=1;i<n;i++)
    {
        fin>>p.x>>p.y;
        total_area += area(ultim,p);
        ultim = p;
    }
    total_area += area(ultim,first);
    total_area /= 2;
}

int main()
{
    ios::sync_with_stdio(false);
    read();
    fout<<setiosflags(ios::fixed)<<setprecision(6)<<(total_area >=0 ? total_area : -total_area);
    return 0;
}