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

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

struct Punct{
    int x, y;
};
vector<Punct> p;
int n;

double AriaPoligon();

int main()
{
    fin >> n;
    p.resize( n + 2 );
    for ( int i = 1; i <= n; ++i )
        fin >> p[i].x >> p[i].y;
    fout << AriaPoligon();
    return 0;
}

double AriaPoligon()
{
    p[n+1] = p[1];
    int i;
    double s = 0;
    for (i = 1; i <= n; i++)
        s += p[i].x * p[i+1].y - p[i].y * p[i+1].x;
    return fabs(s/2);
}