#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct Point{
double x1;
double y1;
};
int N;
Point Arr[100002];
void Read()
{
    int i;
    f>>N;
    for(i=1;i<=N;i++)
        f>>Arr[i].x1>>Arr[i].y1;
}
void Solve()
{
    double S=0;
    int i;
    for(i=1;i<N;i++)
        S+=Arr[i].x1*Arr[i+1].y1;
    S+=Arr[N].x1*Arr[1].y1;
    for(i=1;i<N;i++)
        S-=Arr[i].y1*Arr[i+1].x1;
    S-=Arr[N].y1*Arr[1].x1;
    if(S<0)
        S*=-1;
    S/=2;
    g<<fixed<<setprecision(6)<<S<<"\n";
}
int main()
{
    Read();
    Solve();
    return 0;
}