#include <fstream>
#include <cmath>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");
int n, i;
double arie, xi, xl, x, yi, yl, y;
int main()
{
    in >> n;
    in >> xi >> yi;
    xl = xi; yl = yi;
    for(int i = 1; i <= n; i++) {
        in >> x >> y;
        arie += (xl*y - yl*x);
        xl = x, yl = y;
    }
    arie += (xl*yi - yl*xi);

    out << fabs(arie/2.0);
    return 0;
}