#include <fstream>
#include <iomanip>
using namespace std;

int main()
{
    ifstream in ("aria.in");
    ofstream out ("aria.out");
    double r=0,x,y,a,b,xx,yy;
    int n;
    in>>n>>x>>y;
    xx=x;
    yy=y;
    while(--n)
    {
        in>>a>>b;
        r+=(x*b-a*y);
        x=a;
        y=b;
    }
    r+=(x*yy-xx*y);
    out<<setprecision(5)<<fixed<<r/2<<"\n";
    return 0;
}