#include<fstream>
#include<cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

struct QQ
{
    double x;
    double y;
};
int main()
{
    int n,i;
    long double space=0;
    QQ x1, x2, x3;
    f>>n;
    if(n>2)
	{
        for(i=1; i<=n; ++i)
		{
            if(i==1)
			{
                f>>x1.x>>x1.y;
                x3=x1;
            }
            else
			{
                f>>x2.x>>x2.y;
                    space+=x1.x*x2.y - x2.x*x1.y;
                x1=x2;
            }
        }
        x2=x3;
        space+=x1.x*x2.y - x2.x*x1.y;
    }
    g<<fixed;
    g.precision(5);
    g<<space/2;
return 0;
}