#include <fstream>
#include <cmath>
#include <iomanip>



using namespace std;
struct Point{
    double x, y;
}point[100001];
double area(int n) {
    double x1,x2,x3,y2,y3,s;
    x1 = point[1].x;
    y3 = point[1].y;
    for(int i=2; i<n; ++i) {
        x2 = point[i].x;
        y2 = point[i].y;
        s += (x1 * y2 - x2 * y3);
        x1 = x2;
        y3 = y2;
    }
    s += (x2*point[1].y - point[1].x * y2);
    return s;
}

int main()
{
    int i,n;
    ifstream f("aria.in");
    f >> n;
    for(i = 1 ; i <= n ; ++i)
        f >> point[i].x >> point[i].y;

    ofstream g("aria.out");
    g << fixed << setprecision(6) << area(n);
    return 0;
}