# include <fstream>
# include <cmath>
#define MAX 100005
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct {
    double x, y;
} vec[MAX];
int i,n;
double sol;
int main ()
{

    fin>>n;
    for (i = 0; i < n; ++i)
        fin>>vec[i].x>>vec[i].y;
    vec[n] = vec[0];//pentru a lua si coordonatele ultimului punct cu primul
    for (i = 0; i < n; ++i)
        sol += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
   fout<<abs(sol/2);
   return 0;
}