#include <fstream>
#include <vector>
#define INPUT_FILE "aria.in"
#define OUTPUT_FILE "aria.out"

using Point = std::pair<float, float>;

std::vector<Point> points;

void Read()
{
    std::fstream in(INPUT_FILE, std::ios_base::in);

    if (in.is_open())
    {
        int n;
        float x, y;

        in >> n;

        for (int i = 1; i <= n; i++)
        {
            in >> x >> y;
            points.emplace_back(x, y);
        }
        
        in.close();
    }
}

float CrossProduct(const Point& p1, const Point& p2)
{
    return (p1.first * p2.second - p1.second * p2.first);
}

int main()
{
    Read();

    float area = 0.f;

    for (size_t indx = 0; indx < points.size(); ++indx)
    {
        const Point& firstPoint = points[indx];
        const Point& secondPoint = (indx == points.size() - 1) ? points[0] : points[indx + 1];

        area += CrossProduct(firstPoint, secondPoint);
    }

    std::fstream out(OUTPUT_FILE, std::ios_base::out);

    if (out.is_open())
    {
        out << 0.5f * area << "\n";
        out.close();
    }

    return 0;
}