#include <fstream>
#include <iomanip>

#define Nmax 100100

using namespace std;

struct point {double x, y;} Point[Nmax];
int N;
double Area;

void Solve() {

    Point[N + 1] = Point[1];

    for(int i = 1; i <= N; i++)
        Area += (Point[i].x * Point[i + 1].y - Point[i + 1].x * Point[i].y);

    Area /= 2.0;

    if(Area < 0)
        Area *= -1.0;

}
void Read() {

    ifstream in("aria.in");

    in >> N;
    for(int i = 1; i <= N; i++)
        in >> Point[i].x >> Point[i].y;

    in.close();

}
void Write() {

    ofstream out("aria.out");
    out << fixed << setprecision(6) << Area << '\n';
    out.close();

}
int main() {

    Read();
    Solve();
    Write();

    return 0;

}