#include <fstream>
#include <iomanip>
using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

const int N=100002;
int n;
struct nr{
    long double x,y;
}v[N];

void citire()
{
    cin>>n;
    for(int i=0;i<n;i++)
        cin>>v[i].x>>v[i].y;
}
long double det(nr a, nr b)
{
    return a.x*b.y-a.y*b.x;
}
void arie()
{
    long double s=0;
    for(int i=0;i<n-1;i++)
        s+=det(v[i],v[i+1]);
    s+=det(v[n-1],v[0]);
    cout<<fixed<<setprecision(5)<<abs(s/2);
}
int main()
{
    citire();
    arie();
    return 0;
}