#include <fstream>
#include <iomanip>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

int n;
double x, y, x1, y1;

int main()
{
    in >> n;

    in >> x >> y;
    x1 = x;
    y1 = y;
    int xcurent, ycurent;
    double arie = 0;
    n--;
    while(n--)
    {
        in >> xcurent >> ycurent;

        arie += (double)(xcurent - x)*(ycurent + y);
        x = xcurent;
        y = ycurent;
    }
    arie += (x1 - x)*(y1 + y);
    if(arie < 0)
        arie = -arie;
    out << setprecision(12) << arie / 2;

    return 0;
}