#include <fstream>
#include <iomanip>
#include <cmath>
#define NMAX 100003
using namespace std;
ifstream fi("aria.in");
ofstream fo("aria.out");
struct punct{double x,y ;};
punct p[NMAX];
double aria;
int i,n;
int main()
{
     fi>>n;
     for(i=1; i<=n; i++)
        fi>>p[i].x>>p[i].y;
     p[n+1]=p[1];

     for(i=1;i<=n;i++)
        aria+=(p[i].x*p[i+1].y - p[i].y*p[i+1].x);
    if(aria<0)aria=-aria;

    fo<<fixed<<setprecision(5)<<aria/2.0;
    return 0;
}