#include<fstream>
#include<iomanip>
#define x first
#define y second
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
double Aria;
pair <double,double> p[100010];
int main(){
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>p[i].x>>p[i].y;
    if(n==1||n==2){
        fout<<0;
        return 0;
    }
    for(int i=3;i<=n;i++)
        Aria+=(p[i-1].x-p[1].x)*(p[i].y-p[1].y)-(p[i].x-p[1].x)*(p[i-1].y-p[1].y);
    if(Aria<0)
        Aria*=-1;
    fout<<setprecision(6)<<fixed<<Aria/2;
    return 0;
}