#include <fstream>
#include <iomanip>

using namespace std;


long numberOfPoints, xPoint[1000001], yPoint[1000001];

int main() {
    ifstream inputfile("aria.in");
    ofstream outputfile("aria.out");

    inputfile >>  numberOfPoints;
    for (int i = 1; i <= numberOfPoints; i++)
    {
        int xi,yi;
        inputfile >> xi >> yi;
        xPoint[i] = xi;
        yPoint[i] = yi;
    } // citirea numarului de punct si a coordonatelor acestora

    double area;

    for ( int i = 1; i <= numberOfPoints; i++)
    {
        area = area + (xPoint[i] * yPoint[i+1] - xPoint[i+1] * yPoint[i]);
    }

    area = area + (xPoint[numberOfPoints] * yPoint[1] - xPoint[1] * yPoint[numberOfPoints]);

    outputfile << setprecision(6) << fixed << area / 2.0;

    return 0;
}