#include <iostream>
#include <fstream>
#define nmax 100005
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct poligon
{
    int x,y;
};
poligon a[nmax];
int n;
double ans;
int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
        f>>a[i].x>>a[i].y;
    a[n+1]=a[1];
    for(int i=1;i<=n;i++)
        ans=ans+0.5*(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    g<<ans;
    return 0;
}