#include<fstream>
#include<cmath>
#define Nmax 100099

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

int N;
struct point
{
    long double x,y;

}P[Nmax];

long double Arie(point P[Nmax])
{
    long double aux=0.0;
    for(int i=1;i<=N;i++)
        aux=aux+P[i].x*P[i+1].y-P[i+1].x*P[i].y;
    aux*=0.5;
    return fabs(aux);
}

int main()
{
    f>>N;
    for(int i=1;i<=N;i++) f>>P[i].x>>P[i].y;
    P[N+1]=P[1];
    g.precision(5);
    g<<fixed<<Arie(P)<<'\n';

    f.close();g.close();
    return 0;
}