#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct
{
    double x;
    double y;
}a[100001];
int n,i;
double suma;
int main()
{
    f>>n;
    for (i=1; i<=n; i++) f>>a[i].x>>a[i].y;
    suma=0;
    for (i=1; i<n; i++)
        suma+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    suma+=a[n].x*a[1].y-a[1].x*a[n].y;
    g<<setprecision(6)<<fixed<<(fabs)(suma/2);
    return 0;
}