#include <fstream>
#include <iomanip>

using namespace std;

struct Punct{
  double x, y;
};

int n;
Punct v[100005];

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

void Read(){
  fin >> n;
  for(int i = 1; i <= n; ++i)
    fin >> v[i].x >> v[i].y;
}

void Solve(){
  double s = 0;
  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;
  }
  s /= 2;
  fout << fixed << setprecision(6) << s << "\n";
}

int main(){
  Read();
  Solve();
  return 0;
}