#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
const int NMax = 100000;
pair <double, double> P[NMax + 5];
int main()
{
    int i,n;
    double s=0;
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>P[i].first>>P[i].second;
    P[n+1] = P[1];

    for(i=1;i<=n;i++)
        s=s + P[i].first*P[i+1].second - P[i+1].first*P[i].second;

    s = s / 2;

    fout << fixed << setprecision(6) <<s;
    return 0;
}