#include <bits/stdc++.h>
#define ld long double

using namespace std;

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

struct lol {
	ld x, y;
};

int n;
lol a[100100];

int main() {
	in >> n;
	for (int i = 1; i <= n; i++)
		in >> a[i].x >> a[i].y;
	a[n + 1] = a[1];
	ld ans = 0;
	for (int i = 1; i <= n; i++)
		ans += a[i].x * a[i + 1].y - a[i + 1].x * a[i].y;
	out << fixed << setprecision(6) << abs(ans) / 2.0;
	return 0;
}