#include <fstream>
#include <iomanip>
using namespace std;

int n;
double arie;

struct poligon {
    double x, y;
} p[100002];

void read() {
    double x, y;
    int i;
    ifstream f("aria.in");
    f >> n;
    for (i = 1; i <= n; i++) {
        f >> x >> y;
        p[i].x = x;
        p[i].y = y;
    }
    p[n + 1].x = p[1].x;
    p[n + 1].y = p[1].y;
}

void solve() {
    int i;
    for (i = 1; i <= n; i++)
        arie += 1.0 * (p[i].x * p[i + 1].y - p[i + 1].x * p[i].y) / 2;
}

void output() {
    ofstream g("aria.out");
    g << setprecision(5) << fixed << arie;
    g.close();
}

int main() {
    read();
    solve();
    output();
    return 0;
}