#include <iostream>
#include <fstream>

using namespace std;

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

struct point{
    double x, y;
}ext;

int n;
double arie = 0;

double det(point e, point a, point b){

    return (a.x - e.x) * (b.y - e.y) - (b.x - e.x) * (a.y - e.y);

}

int main() {

    ext.x = -1000007;
    ext.y = -1000007;

    in >> n;

    point a, b;
    point first;
    in >> a.x >> a.y;
    first = a;
    for (int i = 0; i < n; ++i) {
        in >> b.x >> b.y;
        arie += (0.5 * det(ext, a, b));
        a = b;
    }
    arie += (0.5 * det (ext, b, first));

    out << arie;
    return 0;
}