#include <fstream>
#include <iomanip>

using namespace std;

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

int main()
{
    int n;
    float x1,x2,x3,y1,y2,y3;
    float delta=0,sum=0;
    in>>n;
    in>>x1>>y1;
    for(int i = 2; i<=n; i++)
    {
        in>>x3>>y3;
        if(i>=3)
        {
            delta = x1*y2*1 + x2*y3*1 + x3*y1*1 - y1*x2*1 - y2*x3*1 - y3*x1*1;
            sum += delta/2;
        }
        x2=x3;
        y2=y3;
    }
    in.close();
    in.open("poligon.in");
    in>>n;
    for(int i = 2; i<=n-1; i++)
    {
        in>>x2>>y2;
    }
    in>>x3>>y3;
    delta = x1*y2*1 + x2*y3*1 + x3*y1*1 - y1*x2*1 - y2*x3*1 - y3*x1*1;
    sum += delta/2;
    out<<setprecision(15)<<sum;
    return 0;
}