#include <bits/stdc++.h>
using namespace std;
const int N = 1e3 + 10;
struct Point {
    double x, y;
    Point() {}
    Point(double a, double b) : x(a), y(b) {}
    void input() { scanf("%lf%lf", &x, &y); }
    friend Point operator-(Point a, Point b) { return {a.x - b.x, a.y - b.y}; }
    friend double det(Point a, Point b) { return a.x * b.y - a.y * b.x; }
} P[N];

int main() {
    int n; scanf("%d", &n);
    for (int i = 1; i <= n; i++) P[i].input();
    double ans = 0;
    for (int i = 3; i <= n; i++) ans += det(P[i-1] - P[1], P[i] - P[1]);
    if (ans < 0) ans = -ans;
    printf("%.4lf\n", ans / 2.0);
    return 0;
}