#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
long double rez;
struct punct {
    long double x, y;
} v[100010];
int main()
{
    int i,j;
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>v[i].x>>v[i].y;
    }
    rez=0.5*(v[n].x*v[1].y-v[1].x*v[n].y);
    for(i=1;i<n;i++)
    {
        rez=rez+0.5*(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    }
    g<<fixed<<setprecision(6)<<rez;
}