#include <fstream>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

const int Max = 100007;
int n;
double s = 0;

struct coordonate
{
    double x, y;
}v[Max];

int main()
{
    in >> n;
    for(int i = 1; i <= n; ++i)
        in >> v[i].x >> v[i].y;
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;

    for(int i = 1; i <= n; ++i)
    {
        s+= v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    }

    out << s / 2 << "\n";

    return 0;
}