#include <fstream>
#include <iomanip>

using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
int n;
long double arie=0;
struct punct
{
    long double x;
    long double y;
};
punct p[100010];

int main()
{
    fin>>n;
    for(int i=1; i<=n; i++)
        fin>>p[i].x>>p[i].y;
    p[n+1]=p[1];
    for(int i=1; i<=n; i++)
        arie+=(p[i].x*p[i+1].y-p[i+1].x*p[i].y)/2;
    fout<<fixed<<setprecision(15)<<arie;
    return 0;
}