#include <bits/stdc++.h>
#define INFILE "aria.in"
#define OUTFILE "aria.out"
#define x first
#define y second
using namespace std;
ifstream in(INFILE);
ofstream out(OUTFILE);
int N;
typedef pair<double,double> Point;
vector<Point> p;
void Read(){
    in>>N;
    for(int i=1;i<=N;i++){
        double cx,cy;
        in>>cx>>cy;
        p.push_back({cx,cy});
    }
    p.push_back(p[0]);
}
double Arie(){
    double s=0;
    for(int i=0;i<N;i++){
        s+=(p[i].x*p[i+1].y-p[i+1].x*p[i].y);
    }
    s/=2;
    return s;
}
int main(){
    Read();
    out<<fixed<<setprecision(6)<<Arie();
    return 0;
}