#include <fstream>
#include <cmath>
using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
pair < double, double > v[100001];
int n;
double sol;
int main()
{
    f>>n;
    for(int i = 0; i < n; i++)
        f>>v[i].first>>v[i].second;
    v[n] = v[0];
    for (int i = 0; i < n; i++)
        sol += (v[i].first*v[i + 1].second - v[i].second * v[i + 1].first);
    g << fabs(sol / 2.0);
    return 0;
}