#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

struct point {double x, y;} v[100005];
int n,i;
double x=0;

int main()
{
    f>>n;
    for (i=1; i<=n; i++)
        f>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for (i=1; i<=n; i++)
        x+=fabs((v[i].x*v[i+1].y-v[i].y*v[i+1].x));
    i=(int)x;
    n=0; while (i) i/=10, n++;
    g<<setprecision(n+5)<<x/2.0;
    return 0;
}