#include<fstream>

int main()
{
    std::ifstream f("aria.in");

    long double N, x0, y0, x1, y1, x2 = 0, y2 = 0, area = 0;

    f>>N>>x0>>y0;

    x1 = x0; y1 = y0;

    while(--N)
    {
        f>>x2>>y2;

        area += x1 * y2 - x2 * y1;

        x1 = x2; y1 = y2;
    }

    area += x1 * y0 - x0 * y1;

    std::ofstream("aria.out")<<area/2;

    return 0;
}