#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;
    double arcurent;
    while(n-- != 1)
    {
        in >> xcurent >> ycurent;

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

    return 0;
}