#include <fstream>
#define Nmax 100001
using namespace std;
struct punct{double x,y;}
a[Nmax];
int n,i;
double s;
punct p1,p2,po;
int main()
{
    ifstream in("aria.in");
    ofstream out("aria.out");
    in>>n;
    for(i=1;i<=n;++i)
     in>>a[i].x>>a[i].y;
       a[n+1]=a[1];
       for(i=1;i<=n;++i)
       s=s+(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
       if (s<0) s=-s;
       out<<fixed<<s/2;
    return 0;
}