#include <fstream>

using namespace std;
ifstream fcin("aria.in");
ofstream fout("aria.out");
int n,aria;
struct pct
{
    int x,y;
};
pct v[1000001];
int main()
{
    fcin>>n;
    for(int i=1; i<=n; i++)
        fcin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1; i<=n; i++)
        aria+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    fout<<aria/2.0;
    return 0;
}