#include <fstream>
#include <iomanip>
using namespace std;
struct numere{
    long double x,y;
}v[100005];
int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    int n;fin>>n;long double arie=0;long double a,b;
    for(int i=1;i<=n;i++){
        fin>>v[i].x>>v[i].y;
    }
    v[n+1]=v[1];
    for(int i=1;i<=n;i++){
        a=1LL*v[i].x*v[i+1].y;
        b=1LL*v[i+1].x*v[i].y;
        arie+=(long double)a-b;
    }
    arie*=0.5;
    fout<<fixed<<setprecision(6)<<arie<<'\n';
    return 0;
}