#include <fstream>
#include <cmath>

using namespace std;

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

struct coordonate {
    double x, y;
} punct[100005];
int i, n;
double arie;
int main()
{
    in >> n;
    for(int i = 0; i < n; i++) {
        in >> punct[i].x >> punct[i].y;
    }
    punct[n] = punct[0];
    for(int i = 0; i < n; i++) {
        arie += (punct[i+1].x * punct[i].y - punct[i+1].y * punct[i].x);
    }

    out << fabs(arie/2.0);
    return 0;
}