#include <fstream>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n;
struct coord{
    long double x,y;
}v[100003];
long double arie;
long double det(long double x1,long double y1,long double x2,long double y2,long double x3,long double y3){
    return (x2-x1)*(y3-y1)-(x3-x1)*(y2-y1);
}
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>v[i].x>>v[i].y;
    v[0]=v[n];
    for(int i=0;i<n;i++)
        arie+=det(0,0,v[i].x,v[i].y,v[i+1].x,v[i+1].y);
    if(arie<0)
        arie*=(-1);
    cout<<fixed<<setprecision(7)<<arie/2.0;
    return 0;
}