#include <fstream>
#include <iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int main()
{
    int n,i;
    double x1,y1,x2,y2,x,y,s=0;
    in>>n>>x1>>y1;
    x=x1;
    y=y1;
    for(i=2; i<=n; i++)
    {
        in>>x2>>y2;
        s+=x1*y2-x2*y1;
        x1=x2;
        y1=y2;
    }
    s+=x1*y-y1*x;
    out<<fixed<<setprecision(6)<<s/2;
    return 0;
}