// Arie poligon oarecare - O(N)

#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+=(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;
}