#include <fstream>
#include <cmath>
#include <iomanip>
#define nmax 100005
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");

struct poz{double x,y;};
poz v[nmax];
int main()
{int n,i;
double sol;
in>>n;
for(i=1;i<=n;i++)
    in>>v[i].x>>v[i].y;
    sol += v[n].x*v[1].y - v[1].x*v[n].y;
for(i=1;i<=n;i++)
    sol+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
 out<<fixed<<setprecision(5)<<fabs(sol)/2;


    return 0;
}