#include<fstream>
#include<iomanip>
#include<cmath>
using namespace std;
struct point{ double x,y; };
point a[100005];

int main(){
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    int N,i; double sol=0;
    
    cin>>N;
    for(i=1;i<=N;++i)cin>>a[i].x>>a[i].y;
    a[N+1]=a[1];
    for(i=1;i<=N;++i) sol+=(a[i].x*a[i+1].y - a[i+1].x*a[i].y);
    sol/=2.0;
    cout<<setprecision(5)<<sol;
 return 0;  
}