#include <bits/stdc++.h>

using namespace std;

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

int n;
struct nod {
	long double x, y;
} v[100002];


int main() {
	fin >> n;
	for (int i = 1; i <= n; i++)
		fin >> v[i].x >> v[i].y;
	v[n + 1] = v[1];
	long double ans = 0;
	for (int i = 1; i <= n; i++)
		ans += v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
	fout << fixed << setprecision(6) << abs(ans) / 2.0;
	return 0;
}