#include <fstream>
#include <iomanip>
using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
int const NM = 1e5;
pair <double , double> v [1 + NM];
int n;
double base = int(1e6);
double mod (double x){
    return x > 0 ? x : -x;
}
double area (double x , double y , double x1 , double y1){
    return (double)(mod ((double)(x * y1 - x1 * y)));
}
int main()
{
    f >> n;
    double s = 0 ;
    for(int i = 1 ; i <= n ; ++ i)
        f >> v [i] . first >> v [i] . second ;
    for(int i = 1 ; i < n ; ++ i)
        s += area (v [i] . first, v [i] . second , v [1 + i] . first , v [1 + i] . second);
    s += area (v [n] . first , v [n] . second , v [1] . first , v [1] . second);
    s = (double)(s / 2);
    g << fixed << setprecision (5) << s << '\n';
    return 0;
}