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

#define FILE_NAME "aria"
ifstream in (FILE_NAME".in");
ofstream out(FILE_NAME".out");

struct punct
{
    float x, y;
    punct(float _x = 0, float _y = 0)
    {
        x = _x, y = _y;
    }
};
vector < punct > Poligon;
int N;



int main()
{
    in >> N;
    for(int i = 1; i <= N; ++i)
    {
        float x, y;
        in >> x >> y;
        Poligon.push_back(punct(x, y));
    }
    Poligon.push_back(Poligon[0]);

    float arie = 0;
    for(int i = 0; i < N; ++i)
        arie += Poligon[i].x * Poligon[i+1].y - Poligon[i].y * Poligon[i+1].x;
    arie /= 2.0f;
    if(arie < 0)
        arie = -arie;
    out << setprecision(5) << arie;

    return 0;
}