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

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

struct pct{
    double x,y;
} v[100001];

double det(pct a,pct b){
    return a.x*b.y-b.x*a.y;
}

int main()
{
    int n,i;
    double rasp=0;
    cin>>n;
    for(i=1;i<=n;i++) cin>>v[i].x>>v[i].y;
    for(i=1;i<=n;i++){
        rasp+=det(v[i%n+1],v[i]);
    }
    cout<<fixed<<setprecision(5)<<abs(rasp)/2;
    return 0;
}