#include <fstream>
#define PDD pair<double, double>
#define x first
#define y second

using namespace std;

const int N=100005;

ifstream fin("aria.in");
ofstream fout("aria.out");

PDD a[N];

int main()
{
    int n, i;
    double sol=0;
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>a[i].x>>a[i].y;
    }
    a[n+1]=a[1];
    for(i=1;i<=n;i++)
    {
        sol+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    }
    fout<<sol/2;
}