#include <fstream>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct{double x, y;} vec[100005];
int N;
double sol;
int main ()
{
    f>>N;
    for (int i = 0; i < N; ++i)
        f>>vec[i].x>>vec[i].y;
    vec[N] = vec[0];
    for (int i = 0; i < N; ++i)
        sol += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
    g<<sol / 2.0;
}