#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long n,i,j;
double s;
struct ab{
    double x,y;
};
ab v[100001];
int main()
{
    f>>n;
    for (i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    for (i=1;i<=n-1;i++)
        s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
    s+=(v[n].x*v[1].y-v[1].x*v[n].y)/2;
    g.precision(10);
    g<<s;
    f.close();
    g.close();
    return 0;
}