#include <fstream>
#include <cmath>
#include <iomanip>

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

struct ar
{
    int x;
    int y;
} a[100000];

int main()
{
    int n, sum=0;
    fin>>n;
    for(int i=0;i<n;i++)
    {
        fin>>a[i].x>>a[i].y;
        if(i>0)
            sum=sum+((a[i-1].x*a[i].y)-(a[i].x*a[i-1].y));
    }
    sum=(sum+((a[n-1].x*a[0].y)-(a[0].x*a[n-1].y)))/2;
    fout << fixed << setprecision (5) << abs(sum);
        return 0;
}