#include <fstream>
#include <iomanip>

struct Point
{
    int x, y;
};

int main(void)
{
    std::ifstream in("aria.in");
    int nV;
    in >> nV;
    Point *ptrP(new Point[nV + 1]);
    for(int i(0); i < nV; i++)
        in >> ptrP[i].x >> ptrP[i].y;
    in.close();
    double dAns(0.0);
    std::ofstream out("aria.out");
    ptrP[nV] = ptrP[0];
    for(int i(0); i < nV; i++)
        dAns += (ptrP[i].x * ptrP[i + 1].y - ptrP[i + 1].x * ptrP[i].y) / 2.0;
    delete[] ptrP;
    out << std::setprecision(5) << dAns;
    out.close();
    return 0;
}