#include <bits/stdc++.h>

#define ld long double

#define x first

#define y second

using namespace std;

ifstream fi("aria.in");

ofstream fo("aria.out");



int n;

pair <ld, ld> P[100005];



int main()

{

	fi >> n;

	for (int i = 1; i <= n; i++)

	{

		fi >> P[i].x >> P[i].y;

	}

	P[n + 1] = P[1];



	ld rez = 0;

	for (int i = 1; i <= n; i++)

		rez += (P[i].x * P[i + 1].y - P[i].y * P[i + 1].x) / 2.00;



	fo << fixed << setprecision(5) << rez;



	return 0;

}