#include <fstream>
#include <iomanip>
#include <math.h>

struct point
{
    double x;
    double y;
};

point a[1000001];

int main( int argc, char* argv[] )
{
    std::ifstream input("aria.in");
    std::ofstream output("aria.out");

    int N;
    input >> N;
    for ( int i = 0; i < N; ++i )
    {
        double first, second;
        input >> first >> second;
        a[i].x = first; a[i].y = second;
    }
    double sum = 0.0f;

    a[N] = a[0];
    for ( int i = 0; i < N; ++i )
    {
        sum += ( a[i].x * a[i+1].y - a[i+1].x * a[i].y );
    }
    output << std::fixed << std::setprecision(6) << fabs(sum / 2) << "\n";

    input.close();
    output.close();
    return 0;
}