#include <fstream>
#include <algorithm>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

int N;
double aria, x[100005], y[100005];

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

    x[N+1]=x[1]; y[N+1]=y[1];
    for (int i=1; i<=N; ++i)
        aria+=x[i]*y[i+1]-x[i+1]*y[i];
    aria/=2.0;
    g<<fixed<<setprecision(6)<<fabs(aria)<<'\n';
    return 0;
}