#include <fstream>
#include <math.h>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
struct Punct {
              double x;
              double y;
             };             
Punct P[100013];
long  n,i;
double sol;
int main()
{
 cin>>n;
 for(i=1;i<=n;++i) cin>>P[i].x>>P[i].y;
 sol+=P[n].x*P[1].y - P[1].x*P[n].y;
 for(i=1;i<=n;i++) sol+=P[i].x*P[i+1].y-P[i+1].x*P[i].y;
 cout<<fixed<<setprecision(5)<<fabs(sol)/2;
 return 0;
}