#include <iostream>
#include <fstream>
#define nmax
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct Punct{
    int x, y;
};
int determinant(Punct firstPoint, Punct secondPoint)
{
    return firstPoint.x*secondPoint.y-secondPoint.x*firstPoint.y;
}
int main()
{
    int numberPoints;
    int aria=0;
    Punct firstPoint,secondPoint,auxfirstPoint;
    fin>>numberPoints>>firstPoint.x>>firstPoint.y;
    auxfirstPoint=firstPoint;
    for(int i= 2; i <= numberPoints; ++i)
    {
        fin>>secondPoint.x>>secondPoint.y;
        aria+=determinant(firstPoint,secondPoint);
        firstPoint=secondPoint;
    }
     fin >> secondPoint.x >> secondPoint.y;
    aria+=determinant(firstPoint,auxfirstPoint);
    fout<< fixed << setprecision(6) << aria/2;
    return 0;
}