#include <fstream>

using namespace std;

ifstream fin("arie.in");
ofstream fout("arie.out");

int main()
{
    int n;
    double sum = 0, x1, x2, y1, y2, saveX, saveY;

    fin >> n >> saveX>> saveY;
    x1 = saveX;
    y1 = saveY;

    n--;
    while (n) {
        fin >> x2 >> y2;
        sum += x1*y2-x2*y1;
        x1 = x2;
        y1 = y2;
        n--;
    }
    sum += x1*saveY - y2*saveX;

    fout << sum / 2;

    return 0;
}