#include <fstream>
#include <cmath>
#include <iomanip>

using namespace std;

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

int n;

struct punct {
    double x, y;
} puncte[100005];

void citire() {
    f >> n;
    for (int i = 1; i <= n; ++i)
        f >> puncte[i].x >> puncte[i].y;
}

double arie(punct a, punct b, punct c) {
    return abs(a.x * b.y + b.x * c.y + a.y * c.x - b.y * c.x - a.y * b.x - a.x * c.y);
}

double calculare_arie() {
    double suma = 0;
    for (int i = 2; i < n; ++i)
        suma += arie(puncte[1], puncte[i], puncte[i + 1]);
    return suma / 2;
}

int main() {
    citire();
    if (n <= 2) {
        g << 0;
        return 0;
    }
    g << calculare_arie();
    return 0;
}