#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
const int MAXN = 100041;

int n;
double xes[MAXN], yes[MAXN];

void citire()
{
    fin >> n;
    for (int i = 0; i < n; i++) {
        fin >> xes[i] >> yes[i];
    }
    xes[n] = xes[0];
    yes[n] = yes[0];
}

double det(double x1, double y1, double x2, double y2)
{
    return x1 * y2 - x2 * y1;
}

double solve()
{
    double rez = 0;
    for (int i = 0; i < n; i++)
        rez += det(xes[i], yes[i], xes[i+1], yes[i+1]);
    return rez / 2;
}

int main()
{
    citire();
    fout << setprecision(6) << fixed << solve();


    return 0;
}