#include <fstream>
#include <iomanip>
#define MX 100002
using namespace std;

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

struct punct
{
    int x,y;
};
punct a[MX];
int n;
double S;

int main()
{
    int i;

    fin>>n;
    for(i=1; i<=n; i++)
    {
        fin>>a[i].x>>a[i].y;
    }
    a[n+1] = a[1];

    for(i=1; i<=n; i++)
    {
        S += (a[i].x * a[i+1].y - a[i+1].x * a[i].y);
    }
    S = 1. * S / 2;

    fout.fixed;
    setprecision(5);
    fout<<S;

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