#include <fstream>
#include <math.h>
using namespace std;
ifstream fin("arie.in");
ofstream fout("arie.out");
int n,i,s=0,p;
struct coordonate{int x,y;}c[1000];
int main()
{fin>>n;
    for(i=1;i<=n;i++)
      fin>>c[i].x>>c[i].y;
for(i=1;i<=n;i++)
    if(i==n)
     {p=(c[i].x*c[1].y)-(c[i].y*c[1].x);
      s=s+p/2;}
      else
      {p=(c[i].x*c[i+1].y)-(c[i].y*c[i+1].x);
        s=s+p/2;}
fout<<"aria este egala cu "<<s;
    return 0;
}