#include <fstream>
#include <iomanip>
#include <iostream>

using namespace std;

int main()
{
    int n;
    double x[100002];
    double y[100002];

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

    f >> n;
    for(auto i=0; i<n; ++i)
    {
        f >> x[i] >> y[i];
    }

    long double s = 0;

    for(auto i=1; i<n-1; ++i)
    {
        auto d = x[0]   *  y[i]  + y[0]   *  x[i+1]  + x[i] *  y[i+1]  -
                 x[i+1] *  y[i]  - x[0]   *  y[i+1]  - x[i] *  y[0];

        s += d > 0 ? d : -d;
    }

    s = s / 2;

    g << std::setprecision(6) << s;
    return 0;
}