#include <fstream>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int main()
{
    int i,n,x1,y1,x2=1,y2=1,sm;
    double s=0;
    fin>>n;
    for(i=1;i<=(n+1)/2;i++)
    {
        fin>>x1>>y1>>x2>>y2;
        if(i==(n+1)/2&&n%2==1)
        {
            x2=1;
            y2=1;
        }
        s=s+(x1*y2)/2-(x2*y1)/2;
    }
    sm=s*100000;
    fout<<sm/100000<<"."<<sm%100000;
    return 0;
}