#include <fstream>
#define In "aria.in"
#define Out "aria.out"
#define Nmax 100005
using namespace std;
struct Punct
{
    double dx,dy;
};
Punct a[Nmax];
double aria;
int main()
{
    int i,n;
    ifstream fin(In);
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>a[i].dx>>a[i].dy;
    fin.close();
    a[0] = a[n];
    for(i=0;i<n;i++)
        aria+=a[i].dx*a[i+1].dy-a[i].dy*a[i+1].dx;
    if(aria<0)
        aria*=-1;
    aria/=2.0;
    ofstream fout(Out);
    fout<<aria<<"\n";
    fout.close();
    return 0;
}