#include <fstream>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

int n;
double s=0;
long xo,yo,xprev,yprev,x,y;

void adunare()
{
    double l1,l2,l3,arie;
    l1=sqrt((xo-xprev)*(xo-xprev)+(yo-yprev)*(yo-yprev));
    l2=sqrt((xo-x)*(xo-x)+(yo-y)*(yo-y));
    l3=sqrt((x-xprev)*(x-xprev)+(y-yprev)*(y-yprev));
    arie=sqrt((l1+l2+l3)/2*((l1+l2+l3)/2-l1)*((l1+l2+l3)/2-l2)*((l1+l2+l3)/2-l3));
    s=s+arie;


}

int main()
{
f>>n>>xo>>yo>>xprev>>yprev;
for(int i=2;i<n;i++)
{
    f>>x>>y;
    adunare();
    xprev=x;yprev=y;
}
g<<s;
f.close();
g.close();
}