#include <fstream>
#define x first
#define y second
#define pdd pair<double,double>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
pair<double,double> v[100001];
double arie;
int n,i;
double a(pdd c1, pdd c2, pdd c3){
    return (c2.x-c1.x)*(c3.y-c1.y)-(c3.x-c1.x)*(c2.y-c1.y);
}
int main(){
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    for(i=1;i<n-1;i++)
        arie+=a(v[n],v[i],v[i+1]);
    fout<<fixed<<setprecision(5)<<arie/2;
    return 0;
}