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

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

struct Punct
{
  double x,y;
};

const int NMax = 100000;
Punct P[NMax + 5];
int N;
double Sol;

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

void Solve()
{
  for(int i = 1; i <= N; ++i)
    Sol += P[i].x * P[i+1].y - P[i+1].x * P[i].y;
  Sol = Sol / 2;
}

void Print()
{
  fout << fixed << setprecision(6) << Sol << "\n";
}

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