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

struct pct
{
    double x, y;
};
int main()
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    int n, i;
    pct last, ceva;
    cin >> n >> ceva.x >> ceva.y;
    last = ceva;
    double ans = 0;
    for (i = 2; i <= n; i++)
    {
        double a, b;
        cin >> a >> b;
        ans += b * last.x - a * last.y;
        last = {a, b};
    }
    ans += ceva.y * last.x - ceva.x * last.y;
    ans /= 2;
    cout << ans;
}