#include <fstream>
std::ifstream fin("aria.in");
std::ofstream fout("aria.out");
class point{
    public:
    double x, y;
};
point arr[100001];
int main(){
    int n;
    fin >> n;
    for(int i = 1; i <= n; i++){
        fin >> arr[i].x >> arr[i].y;
    }
    double sum = 0;
    arr[n + 1] = arr[1];
    for(int i = 1; i <= n; i ++){
        sum += ((arr[i].x * arr[i + 1].y) - (arr[i + 1].x * arr[i].y));
    }
    fout << sum / 2.0;
}