#include <iomanip>
#include <cstdio>
#include <fstream>
using namespace std;
const int NMAX = 100005;
struct Punct {
    long double x, y;
};
Punct v[NMAX];
Punct init;
long double aflare_arie(Punct a, Punct b, Punct c) {
    return ((a.x - b.x) * (a.y - c.y) - (a.y - b.y) * (a.x - c.x)) / 2.0;
}
ifstream in ("aria.in");
ofstream out ("aria.out");

int main() {
    int n;
    in >> n;
    for(int i = 1; i <= n; i++) {
        in >> v[i].x >> v[i].y;
    }
    v[n + 1] = v[1];
    init.x = init.y = 0;
    long double sol = 0;
    for(int i = 1; i <= n; i++) {
        long double solc = aflare_arie(init, v[i], v[i + 1]);
        sol += solc;
    }
    out << fixed << setprecision(6) << sol << "\n";
    return 0;
}