#include <fstream>
#include <cmath>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
float x[21],y[21];
int main()
{
    float delta,s=0;
    int i,n;
    bool k=true;
    in>>n;
    for(i=1;i<=n;i++)
        in>>x[i]>>y[i];
    for(i=1;i<=n-2;i++)
    {
        delta=x[i]*y[i+1]+x[i+1]*y[i+2]+x[i+2]*y[i]-y[i+1]*x[i+2]-y[i+2]*x[i]-y[i]*x[i+1];
        delta=delta/2;
        s+=delta;

    }
    out<<s;
    return 0;
}