#include<fstream>
#include<cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
float s;
struct {float x,y;} v[100005];
int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    v[n+1].x=v[1].y;
    v[n+1].y=v[1].y;
    for(int i=1;i<=n;i++)
        s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    g<<fabs(s/2.0);
    g.close();
    return 0;
}