#include <iostream>
#include <fstream>
#include <math.h>
#include <iomanip>
using namespace std;
int main()
{
double a[100002],v[100002];
int n;
double s;
ifstream fin("aria.in");
ofstream fout("aria.out");
    int i;
    cin>>n;
  for(i=1;i<=n;++i)
   cin>>a[i]>>v[i];
    a[n+1]=a[1];
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
    {
        s=s+a[i]*v[i+1]-a[i+1]*v[i];
 
    }
 
    fout<<fixed<<setprecision(5)<<s/2;
    return 0;
}