#include<fstream>
#include<iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double s;
struct vectorul
{
    double x,y;
};
vectorul a[100002];
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
     f>>a[i].x>>a[i].y;
    a[n+1].x=a[1].x;
    a[n+1].y=a[1].y;
    for(i=2;i<=n+1;i++)
        s+=a[i-1].x*a[i].y-a[i].x*a[i-1].y;
    if(s<0)
    s=s*(-1);
    g<<fixed<<setprecision(5)<<s/2;
    return 0;
}