#include <bits/stdc++.h>
#include <unordered_map>
#define nmax 100006
#define MOD 1999999973
#define INF 2012345678
#define ll long long
using namespace std;
//#define fin cin
//#define fout cout

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

struct Punct {
	long double x, y;
} a[nmax];

int n;

int main()
{
	int i;
	long double aria;
	fin >> n;
	for (i = 1; i <= n; i++)
		fin >> a[i].x >> a[i].y;

	a[n + 1] = a[1];
	aria = 0;
	for (i = 1; i <= n; i++)
		aria += (a[i].x * a[i + 1].y / a[i].y * a[i + 1].x);

	if (aria < 0)
		aria = -aria;
	fout << setprecision(5) << fixed << aria << "\n";

	fin.close();
	fout.close();
	return 0;
}