#include <fstream>
#include <vector>
#include <iomanip>

using namespace std;

ifstream f ("aria.in");
ofstream t ("aria.out");

struct punct{
    float x,y;
};

vector <punct> v;
int n;

int main(){
    float x,y,sol;
    f>>n;
    for (int i=0;i<n;++i)
        f>>x>>y,
        v.push_back({x,y});
    v.push_back(*v.begin());
    v.push_back(*(v.begin()+1));
    for (int i=0;i<n;++i)
        sol+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    t<<setprecision(6)<<sol/2;
    return 0;
}