#include <fstream>
#include <iomanip>
#define DIM 100002
using namespace std;

ifstream fin ("aria.in");
ofstream fout ("aria.out");
double sol;
int n;
pair <double,double> v[DIM];
double aria (pair <double,double> x, pair<double,double> y, pair <double,double> z){
    return (y.first-x.first) * (z.second - x.second) - (z.first - x.first) * (y.second - x.second);
}

int main (){

    fin>>n;
    for (int i=1;i<=n;i++)
        fin>>v[i].first>>v[i].second;
    v[0] = v[n];
    for (int i=0;i<n;i++){
        sol += aria (v[0],v[i],v[i+1]);
    }
    fout<<setprecision (6)<<fixed << sol/2;



    return 0;
}