#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;

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

struct punct{
double x,y;
};
double Arie(int n, punct p[]);
punct p[105000];
double s;
int main()
{
    int n;
    fin >> n;
    for(int i = 1; i <= n; i++)
        fin >> p[i].x >> p[i].y;
    fout << fixed << setprecision(5) <<  Arie(n, p);
    return 0;
}
double Arie(int n,punct p[])
{
    p[n + 1] = p[1];
    int i;
    for(i = 1; i <= n; i++)
        s += p[i].x * p[i + 1].y - p[i + 1].x * p[i].y;
    return fabs(s/2.0);
}