#include <fstream>
#include <iomanip>

using namespace std;

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

struct punct
{
    int x, y;
} v[100005];

int main()
{
    int n;
    double A;
    fin >> n;
    for(int i = 1; i <= n; ++i) fin >> v[i].x >> v[i].y;
    fin.close();
    v[n+1] = v[1];
    for(int i = 1; i <= n; ++i)
        A += (v[i].x*v[i+1].y - v[i+1].x*v[i].y);
    fout << setprecision(5) << fixed << A/2.0;
    fout.close();
    return 0;
}