#include<fstream>
#include<cmath>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
struct xy{
    int x,y;
};
int n;
xy a[100010];
double rez;
int main(){
cin>>n;
cout.precision(6);
for(int i=0;i<n;i++)cin>>a[i].x>>a[i].y;
a[n]=a[0];
for(int i=0;i<n;i++){
    rez+=a[i].x*a[i+1].y-a[i].y*a[i+1].x;
}
cout<<fixed<<abs(rez)/2;
}