package CW2010;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class prob11
{

    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        int numV = scan.nextInt();
        List<double[]> vert = new ArrayList<>();
        for (int i = 0; i < numV; i++)
        {
            vert.add(new double[] { scan.nextDouble(), scan.nextDouble()});
        }

        int currentV = 0;
        double area = 0;
        while (true)
        {
            if (vert.size() <= 2)
                break;
            double[] v1 = vert.get(currentV % (vert.size() ));
            double[] v2 = vert.get(((currentV+1) % (vert.size() )));
            double[] v3 = vert.get(((currentV+2) % (vert.size() )));
            double a = Math.sqrt(Math.pow(v2[0] - v1[0], 2)
                    + Math.pow(v2[1] - v1[1], 2));
            double b = Math.sqrt(Math.pow(v3[0] - v2[0], 2)
                    + Math.pow(v3[1] - v2[1], 2));
            double c = Math.sqrt(Math.pow(v1[0] - v3[0], 2)
                    + Math.pow(v1[1] - v3[1], 2));
            double p = (a + b + c) / 2;

            area += Math.sqrt(p * (p - a) * (p - b) * (p - c));
            currentV += 1;
            vert.remove(((currentV) % (vert.size())));
        }
        System.out.println(area);

    }

}