#include <fstream>
#include <iomanip>
using namespace std;
const int NMAX = 100000;
double x,y,ans;
struct TIP
{
    double A,B;
}V[NMAX+5];
int N;
int main()
{
    ifstream fi ("aria.in");
    ofstream fo ("aria.out");
    fi >> N;
    for(int i=1;i<=N;++i)
    {
        fi >> x >> y;
        V[i].A = x;
        V[i].B = y;
    }
    fi.close();
    V[N+1].A=V[1].A;
    V[N+1].B=V[1].B;
    for (int i=1;i<=N;++i)
    {
        x = V[i].A * V[i+1].B;
        y = V[i+1].A * V[i].B;
        ans += x - y;
    }
    fo << setprecision(6) << fixed << 0.5 * ans << '\n';
    fo.close();
    return 0;
}