#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream fi("aria.in");
ofstream fo("aria.out");
const int NMAX=1e5+5;
struct punct {
    long double x,y;
};
int n;
long double arie;
punct V[NMAX];
int main()
{
    fi>>n;
    for(int i=1;i<=n;i++)
    {
        fi>>V[i].x>>V[i].y;
        V[i].x+=1000001.0;
        V[i].y+=1000001.0;
    }
    V[n+1]=V[1];
    for(int i=1;i<=n;i++)
        arie+=(V[i+1].x-V[i].x)*(V[i+1].y+V[i].y);
    if(arie<0) arie=-arie;
    fo<<fixed<<setprecision(6)<<arie/2;
    fi.close();
    fo.close();
    return 0;
}