#include <fstream>

using namespace std;

int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    // (baza mare +baza mica)*inaltimea pe 2
    double s=0;
    double n,x1,y1,x2,y2,x3,y3,i;
    f>>n;
    f>>x1;
    f>>y1;
    x2=x1;
    y2=y1;
    for(i=2;i<=n;i++)
    {
        f>>x3;
        f>>y3;
        s+=((y3+y2)*(x3-x2)/-2);
        x2=x3;
        y2=y3;
       //g<<s<<" ";
    }
    s+=((x2-x1)*(y2+y1)/2);
    g<<s;
    f.close();
    g.close();
    return 0;
}