#include <fstream>
#include <iomanip>
#include <cmath>

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

struct
{
    double x,y;
}v[100002];
int main()
{
    int n;
    f>>n;
    for(int i=1;i<=n;++i)
        f>>v[i].x>>v[i].y;
    double arie;
    v[n+1]=v[1];
    for(int i=1;i<=n;++i)
        arie=arie+v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    if(arie<0)
        arie=-arie;
    arie/=2;
    g<<fixed<<setprecision(7)<<arie;
}