#include <fstream>
#include<iomanip>
#define NMAX 100005
using namespace std;

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

long n;
double s;

struct pct{ double x,y;}V[NMAX];

int main()
{
     f>>n;
     for (long i=1;i<=n;i++)
     f>>V[i].x>>V[i].y;

     V[n+1].x=V[1].x;
     V[n+1].y=V[1].y;

     for (long i=1;i<=n;i++)
     s+=V[i].x*V[i+1].y-V[i+1].x*V[i].y;


      g<<setprecision(6)<<fixed<<s/2;
return 0;}