#include<fstream>
#include<iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
struct puncte
{
    double x,y;
};
puncte v[100002];
int n;
double arie;
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1;i<=n;i++)
        arie+=(v[i].x*v[i+1].y)-(v[i+1].x*v[i].y);
    cout<<fixed<<setprecision(6)<<arie/2;
}