#include <fstream>
#include <cmath>
#include <iomanip>
#define nmax (int)(1e5+1)
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n;
long double arie;
struct punct{
    long double x,y;
}p[nmax];
long double det(punct a,punct b,punct c){
    return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);
}
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>p[i].x>>p[i].y;
    p[0]=p[n];
    for(int i=0;i<n;i++)
        arie+=det({0,0},p[i],p[i+1]);
    cout<<fixed<<setprecision(5)<<abs(arie)/2.0;
    return 0;
}