#include <fstream>
#include <iomanip>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    int n, i;
    long double x1, y1, x = 0, y = 0, xx, yy;
    long double aria = 0;
    fin >> n >> x1 >> y1;
    xx = x1;
    yy = y1;
    for (i = 2; i<=n; i++)
    {
        fin >> x >> y;
        aria = aria + xx*y - x*yy;
        xx = x;
        yy = y;
    }
    aria = aria + x*y1 - x1*y;
    const long double imp = 2, minus = -1;
    aria = aria / imp;
    if (aria < 0)
        aria = aria * minus;
    fout << fixed << setprecision(6) << aria;
    return 0;
}