#include <fstream>
#include <iomanip>
#include <math.h>
#define NMax 100005
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long double aria;
int n;
struct punct {
    double x;
    double y;
}p[NMax];
int main()
{
    f>>n;
    for (int i=1; i<=n; i++)
        f>>p[i].x>>p[i].y;
    aria += p[n].x*p[1].y - p[1].x*p[n].y;
    for (int i=1; i<=n; i++)
        aria += p[i].x*p[i+1].y - p[i+1].x*p[i].y;
    g<<fixed<<setprecision(5)<<fabs(aria)/2;
    return 0;
}