#include<fstream>
#include<iomanip>
#define x first
#define y second
using namespace std;
const int Maxx=1e5+1;
ifstream fin("aria.in");
ofstream fout("aria.out");
pair<double,double> A[Maxx];
int main() {
    int points;
    fin>>points;
    for (int i=1;i<=points;++i){
        fin>>A[i].x>>A[i].y;
    }
    double aria=0;
    A[points+1]=A[1];
    for (int i=1;i<=points;++i){
        aria+=(A[i].x*A[i+1].y-A[i+1].x*A[i].y);
    }
    fout<<setprecision(6)<<fixed<<(aria/2.0);
    return 0;
}