#include <fstream>
#define N_MAX 100005
using namespace std;

ifstream in { "aria.in" };
ofstream out { "aria.out" };

struct {
    double x, y;
} a[N_MAX];

int main() {
    int n; in >> n;
    for (int i { 1 }; i <= n; ++i)
        in >> a[i].x >> a[i].y;
    
    a[n + 1] = a[1];
    
    long double A { 0 };
    for (int i { 1 }; i <= n; ++i)
        A += a[i].x * a[i + 1].y - a[i + 1].x * a[i].y;
    
    out << A / 2;
}