#include <fstream>
#include <iomanip>

using namespace std;

struct point
{
    double x;
    double y;
};

int N;
point* P;
double A = 0;

void Read()
{
    ifstream f("aria.in");
    f>>N;
    P = new point[N];
    double x, y;
    for (int i=0;i<N;i++)
    {
        f>>x>>y;
        P[i] = {x, y};
    }
}

void Area()
{
    for (int i=0;i<N;i++)
    {
        A += (P[i].x*P[(i+1)%N].y - P[(i+1)%N].x*P[i].y)/2;
    }
}

void Write()
{
    ofstream g("aria.out");
    g<<fixed<<setprecision(5)<<max(A, -A)<<'\n';
}

int main()
{
    Read();
    Area();
    Write();
}