#include <fstream>
#include<iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct
{
    float x,y;
};
punct a[100004];
double aria;
int n,i;

//double x[100004],y[100004],aria;
int main(void)
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>a[i].x>>a[i].y;
    a[n+1].x=a[1].x;
    a[n+1].y=a[1].y;
   // for (i=1;i<=n;i++)
     // fscanf(f,"%lf%lf",&x[i],&y[i]);
    //x[n+1]=x[1];
    //y[n+1]=y[1];
    for (i=1;i<=n;i++)
        aria=aria+a[i].y+a[i].x*a[i+1].y+a[i+1].x-a[i+1].x*a[i].y-a[i+1].y-a[i].x;
       // aria=aria+x[i]*y[i+1]-x[i+1]*y[i];
    aria/=2;
    if (aria<0) aria*=-1;
   // fprintf(g,"%.5lf",aria);
   g<<fixed<<setprecision(5)<<aria;
    return 0;
}