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

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

struct Punct{
    double x, y;
} A, B, X;

int n;

double AriaPoligon();

int main()
{
    fin >> n;
    fin >> A.x >> A.y;
    X = A;
    fout << AriaPoligon();
    return 0;
}

double AriaPoligon()
{
    int i;
    double s = 0;
    for (i = 1; i < n; i++)
    {
        fin >> B.x >> B.y;
        s += A.x * B.y - A.y * B.x;
        A = B;
    }
    s += A.x * X.x - A.y * X.x;
    return fabs(s/2);
}