#include <fstream>
#include <iomanip>
#define VAL 100005

using namespace std;

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

struct punct
{
    long double x;
    long double y;
};

punct v[VAL];

int N, i;
long double Arie;

int main()
{
    fin >> N;
    for (i=1; i<=N; i++)
      fin >> v[i].x >> v[i].y;
    v[N+1]=v[1];
    for (i=1; i<=N; i++)
      Arie+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    if (Arie<0)
      Arie*=-1;
    fout << fixed << setprecision(7) << Arie / 2 << '\n';
    fin.close();
    fout.close();
    return 0;
}