#include <iostream>
#include <cmath>
#include <cstdio>

using namespace std;
const int L = 100000;

int n;
double arie = 1;

void arie_triunghi()
{

}

int lungime(int x, int y)
{
    return sqrt(x*x + y*y);
}

void read()
{
    scanf("%d", &n);
    for(int i=0; i<n/2; i++)
    {
        double x1, y1, x2, y2;
        scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
        arie *= 0.5*(x1*x2 - x1*y2);
    }
}

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    read();
    cout<<arie;
}