#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;

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

const int maxN = 100001;
struct punct {
    double x, y;
};
punct v[maxN];

int main()
{
    int n;
    long double s = 0;
    in >> n;
    for(int i = 1; i <= n; i++)
        in >> v[i].x >> v[i].y;
    v[0].x = v[n].x, v[0].y = v[n].y;
    for(int i = 1; i <= n; i++)
        s += (v[i-1].x * v[i].y - v[i+1].x * v[i-1].y);
    out << fixed << setprecision(5) << s/2;
    return 0;
}