#include <fstream>
#include <iomanip>

using namespace std;

const int N = 100000;

double x[N], y[N];

int main()
{
    ifstream in("aria.in");
    ofstream out("aria.out");
    int n;
    long double aria = 0;
    in >> n >> x[0] >> y[0];
    for (int i = 1; i < n; i++)
    {
        in >> x[i] >> y[i];
        aria += (x[i] - x[i-1]) * (y[i] + y[i-1]);
    }
    in.close();
    aria += (x[0] - x[n-1]) * (y[0] + y[n-1]);
    if (aria < 0)
    {
        aria = -aria;
    }
    out << setprecision(6) << fixed << aria / 2;
    out.close();
    return 0;
}