#include <fstream>
#include <cmath>
#include <iomanip>
#define DIM 100002
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct{double x,y;}v[DIM];
double arie;
int n,i,j;
double aria(punct p1,punct p2,punct p3){
    return ((p2.x-p1.x)*(p3.y-p1.y)-(p3.x-p1.x)*(p2.y-p1.y))/2.0;
}
int main(){
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    for(i=2;i<n;i++)
        arie+=aria(v[1],v[i],v[i+1]);
    fout<<setprecision(1)<<fixed<<abs(arie);
    fin.close();fout.close();
    return 0;
}