#include <fstream>
#include <iomanip>
#include <cmath>
#define Nmax 100005
using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int n,i;
double sol;
struct punct {int x;int y;};
punct a[Nmax]; //ora 00:39. Ma doare gatul ,mi s-au terminat nectarinele si mii somn.

int main()
{
    f>>n;
    for (i=1;i<=n;i++)
     f>>a[i].x>>a[i].y;
    sol+=a[n].x*a[1].y-a[n].y*a[1].x;
    for (i=1;i<=n;i++)
    sol+=a[i].x*a[i+1].y-a[i].y*a[i+1].x;
    g<<fixed<<setprecision(5)<<fabs(sol)/2;
    return 0;
}