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

using namespace std;

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

int N, i;
double x[VAL], y[VAL];
double ANS;

int main()
{
    fin >> N;
    for (i=1; i<=N; i++)
      fin >> x[i] >> y[i];
    x[0]=x[N];
    y[0]=y[N];
    x[N+1]=x[1];
    y[N+1]=y[1];
    for (i=1; i<=N; i++)
    {
        ANS+=x[i]*y[i+1];
        ANS-=x[i]*y[i-1];
    }
    ANS=max(ANS, -ANS);
    fout << fixed << setprecision(8) << ANS / 2 << '\n';
    fin.close();
    fout.close();
    return 0;
}