#include <fstream>
#define NMAX 100010

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

long long n,i;
double x[NMAX],y[NMAX];
double s=0;

int main()
{
    in>>n;
    for(i=1;i<=n;i++)
    {
        in>>x[i]>>y[i];
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
    {
        s=s+x[i]*y[i+1]-y[i]*x[i+1];
    }
    out<<s/2;
    in.close();
    out.close();
    return 0;
}