#include <iostream>
#include <fstream>
#include <cstring>
#include <iomanip>

using namespace std;

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

int n;

int main() {
    fin >> n;
    long double aria = 0;
    long double x0, y0;
    fin >> x0 >> y0;
    long double x_anterior = x0;
    long double y_anterior = y0;
    for (int i = 1; i < n; i++) {
        long  double xi, yi;
        fin >> xi >> yi;
        //aria += (x_anterior + xi) * (y_anterior + yi);
        aria += x_anterior * yi - xi * y_anterior;
        x_anterior = xi;
        y_anterior = yi;
    }
    //aria += (x_anterior - x0) * (y_anterior - y0);
    aria += x_anterior * y0 - x0 * y_anterior;
    fout << setprecision(5) << fixed << aria / 2 << "\n";
    return 0;
}