#include <fstream>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
double x[100006],y[100006];
int i,n;
double aria;
int main()
{  int i;
    fin>>n;
    for(i=1;i<=n;++i)
    { fin>>x[i];
        fin>>y[i];
    }
  x[n+1]=x[1];
  y[n+1]=y[1];
   for(i=1;i<=n;++i)
    aria+=(x[i]*y[i+1]-x[i+1]*y[i])/2;
    fout<<aria;
    return 0;
}