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

#define x first
#define y second

typedef pair<double,double> Point;

Point start, p1, p2;
double area;
int N, i;

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

    fin >> N;

    fin >> start.x >> start.y;
    p1 = start;
    i = 2;
    do
    {
        fin >> p2.x >> p2.y;
        area += (p1.x * p2.y) - (p1.y * p2.x);
        p1 = p2;
        ++i;
    } while(i <= N);

    area += (p1.x * start.y) - (p1.y * start.x);

    area /= 2;

    fout << setprecision(5) << area;

    fin.close();
    fout.close();
    return 0;
}