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

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

struct Punct{
    double x,y;
};

Punct v[100002];

int main()
{
    int n;
    double s = 0;
    fin >> n;
    for(int i = 1; i <= n; ++i)
        fin >> v[i].x >> v[i].y;
    v[n+1]=v[1];
    for(int i = 1; i <= n; ++i){
        s += v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    }
    fout << fixed << setprecision(6) << s << "\n";
    return 0;
}