#include <iomanip>
#include <cmath>
#include <fstream>
#define per pair<double,double >
#define DN 100005
#define x first
#define y second
using namespace std;

double aria;
per p[DN];

int main()
{
    int n;
    ifstream f("aria.in");
    ofstream g("aria.out");
    f>>n;
    for(int i=1;i<=n;++i)
        f>>p[i].x>>p[i].y;

    p[n+1]=p[1];
    for(int i=1;i<=n;++i)
        aria+=(p[i].x*p[i+1].y - p[i].y*p[i+1].x);

    g<<fixed<<setprecision(6)<<(fabs(aria)/2.0);
    return 0;
}