#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int NMAX=1000003;
int n;
double arie;
double x[NMAX],y[NMAX];
int main()
{   f>>n;
    int i;
    for(i=1;i<=n;i++) f>>x[i]>>y[i];
    x[n+1]=x[1]; y[n+1]=y[1];
    for(i=1;i<=n;i++) arie+=x[i]*y[i+1]-x[i+1]*y[i];
    arie/=2.0;
    if(arie<0) arie=-arie;
    g<<arie; g.close();
    return 0;
}