#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream f("aria.in"); ofstream g("aria.out");
int n,i;
long double Arie = 0.0;
struct p{long double x, y;} P[100004];
int main()
{   f>>n;
    for(i=1;i<=n;++i) f>>P[i].x>>P[i].y;
    P[n+1]=P[1];
    for(i=1;i<=n;++i)
        Arie+=(P[i].x*P[i+1].y-P[i+1].x*P[i].y);
    Arie/=2.0;
	if(Arie<0) Arie=-Arie;
    g<<fixed<<setprecision(5)<<Arie<<'\n';
    return 0;
}