#include<fstream>
#include<cmath>
using namespace std;
double sum;
int i,n;
struct punct
{
    double x,y;
}a[100001];
int main()
{
    fstream f("aria.in",ios::in);
    fstream g("aria.out",ios::out);
    f>>n;
    for (i=0;i<n;++i)
    f>>a[i].x>>a[i].y;
    a[n]=a[0];
    for(i=0;i<n;++i)
        sum+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
    g<<fabs(sum/2.0);
    return 0;
}