#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

struct Punct
{
    int x,y;
}p[100001];

long double arie;
int n;

int main()
{

    fin>>n;
    for(int i = 1; i<= n; i++ )
    {
        fin>>p[i].x>>p[i].y;
    }
    p[0]=p[n];
    for(int i = 0; i< n; i++ )
    {
        arie+=p[i].x*p[i+1].y-p[i+1].x*p[i].y;
    }
    fout<<setprecision(7)<<fixed<<arie/2;
    fin.close();
    fout.close();
    return 0;
}