#include <fstream>
#include <iomanip>
#define DIM 100011
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
pair<double,double> v[DIM];
double sol;

inline double det(pair<double,double> &a,pair<double,double> &b){
    return (a.first*b.second)-(a.second*b.first);
}

int main(void){
    register int i,j,x,y;

    f>>n;
    for(i=1;i<=n;i++){
        f>>x>>y,v[i]=make_pair(x,y);
    }

    v[n+1]=v[1];
    for(i=1;i<=n;i++)
        sol+=det(v[i],v[i+1]);
    sol/=2;
    g<<setprecision(5)<<fixed<<sol;
    f.close();
    g.close();
    return 0;
}