#include <fstream>
#include <iomanip>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
typedef long double Real;
int n;
Real a1,b1,a2,b2,a3,b3,aria;
int main()
{
    f>>n>>a1>>b1>>a2>>b2;
    for(int i=3;i<=n;i++)
    {
        f>>a3>>b3;
        aria+=(a1*b2+a2*b3+a3*b1-b1*a2-b2*a3-b3*a1)/2;
        a2=a3;b2=b3;
    }
    g<<fixed<<setprecision(10)<<aria<<'\n';
    return 0;
}