#include <fstream>

using namespace std;
#define Nmax 100000

ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct{
    double x,y;
}a[Nmax];
double aria;
int n;
int main(){

    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>a[i].x>>a[i].y;
    }
    a[n+1]=a[1];
    for(int i=1;i<=n;i++)
        aria+=a[i].x*a[i+1].y-a[i].y*a[i+1].x;

    aria/=2;
    fout<<aria;
    return 0;
}