#include <bits/stdc++.h>
#define nmax 100007

using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");

int n;
int x[nmax], y[nmax];
long double rasp;

void Citire()
{
    int i;
    fin >> n;
    for (i = 1; i <= n; i++)
        fin >> x[i] >> y[i];
}

int Arie(int x1, int y1, int x2, int y2)
{
    return (x1 * y2 - x2 * y1);
}

void Rezolvare()
{
    int i;
    for (i = 2; i <= n; i++)
        rasp += 1.0 * Arie(x[i-1], y[i-1], x[i], y[i]) / 2;
    rasp += 1.0 * Arie(x[n], y[n], x[1], y[1]) / 2;
    fout << setprecision(7) << fixed << rasp;
}

int main()
{
    Citire();
    Rezolvare();

    fin.close();
    fout.close();
    return 0;
}