#include <fstream>
#include <cmath>
#include <iomanip>

using namespace std;

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

struct arie
{
    double x, y;
};
arie v[100001];

int i,n;
double s=0;

int main()
{
    f>>n;
    for (i=1; i<=n; ++i)
    f>>v[i].x>>v[i].y;

    v[n+1]=v[1];

    for (i=1; i<=n; ++i)
    s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);

    g<<setprecision(5)<<abs(s/2);

    return 0;
}