#include <fstream>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int n;
double x,y,xa,ya,s,x1,y1;

int main()
{
    int i;
    f>>n;
    f>>xa>>ya;
    x1=xa;
    y1=ya;
    for(i=1;i<n;++i)
    {
        f>>x>>y;
        s=s+xa*y-ya*x;
        xa=x;
        ya=y;
    }
    s=s+x*y1-y*x1;
    s/=2;
    g<<s<<'\n';
    f.close(); g.close();
    return 0;
}