#include <fstream>
#include <iomanip>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
double a;
int i,n;
struct punct
{
    double x,y;
}v[100100];
int mod (int x)
{
    if(x<0)
        return -x;
    return x;
}
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    for(i=2;i<n;i++)
    {
        a+=mod(v[1].x * v[i].y + v[i].x * v[i+1].y + v[i+1].x *v[1].y - v[i+1].x * v[i].y - v[i].x * v[1].y - v[1].x * v[i+1].y)/2.0;
    }

    fout<<fixed<<setprecision(5)<<a;
    return 0;
}