#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
const int eps = 1.e-14;
int main()
{   ifstream fin("aria.in");
    ofstream fout("aria.out");
    long double x1,y1,x2,y2,a=0.0,x3,y3;
    int i,n;
    fin>>n;
    fin>>x1>>y1;
    x3=x1;
    y3=y1;
    for(i=2; i<=n;i++){
        fin>>x2>>y2;
        a+=(x1*y2)-(x2*y1);
        x1=x2;
        y1=y2;
    }
    a+=(x1*y3)-(x3*y1);
    a=a/2.0;
    a=fabs(a);
     fout<<setprecision(7)<<a;
    return 0;
}