#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

pair <double, double> coord[100005];
double sum=0.0;
int n;

int main()
{
    f>>n;
    for (int i=1; i<=n; i++)
      f>>coord[i].first>>coord[i].second;

    coord[n+1]=coord[1];

    for (int i=2; i<=n+1; i++)
      sum+=coord[i-1].first*coord[i].second-coord[i].first*coord[i-1].second;

    g<<fixed<<setprecision(6)<<sum*0.5;
    return 0;
}