#include <fstream>
#include <iomanip>

using namespace std;

int main() {
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    
    int n;
    fin >> n;
    
    long double x1, y1, x2, y2;
    fin >> x1 >> y1;
    
    long double first_x = x1, first_y = y1;
    long double area = 0;
    
    for (int i = 1; i < n; ++i) {
        fin >> x2 >> y2;
        area += x1 * y2 - x2 * y1;
        x1 = x2;
        y1 = y2;
    }
    
    area += x1 * first_y - first_x * y1;
    
    if (area < 0) area = -area;
    area /= 2.0L;
    
    fout << fixed << setprecision(5) << area << "\n";
    
    return 0;
}