#include <fstream>
using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

int main()
{
    int n;
    cin >> n;
    double x, y, a = 0, b, c;
    cin >> b >> c;
    for(int i = 2; i <= n; i++)
    {
        cin >> x >> y;
        a += b * y - c * x;
        b = x;
        c = y;
    }
    cout.precision(5);
    cout << fixed << abs(a) / 2;
    return 0;
}