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

const int kNMax = 100010;
struct elem {double x, y;} punct[kNMax];
int n;
double aria;

void Citire() {
    ifstream in("aria.in");
    in >> n;
    for (int i = 1; i <= n; ++i)
        in >> punct[i].x >> punct[i].y;
    in.close();
}

void Solve() {
    punct[n + 1] = punct[1];
    for (int i = 1; i <= n; ++i)
        aria += punct[i].x * punct[i + 1].y - punct[i].y * punct[i + 1].x;
    aria /= 2;
    if (aria < 0)
        aria = -aria;
}

void Afisare() {
    ofstream out("aria.out");
     out << fixed << setprecision(10) << aria << '\n';
    out.close();
}

int main () {
    Citire();
    Solve();
    Afisare();
    return 0;
}