#include <iostream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <set>
#include <vector>
#include <map>
#include <queue>
#include <stack>
#include <utility>
#include <string>
#include <cstring>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <limits>
#include <sstream>
#include <deque>
#include <bitset>
#include <complex>
#include <functional>
#include <memory>
#include <numeric>
#define x first
#define y second
typedef std::pair<int, int> pii;

using namespace std;



int main () {
	ifstream fin("aria.in");
	ofstream fout("aria.out");
	int n, x[100002], y[100002];
	float sol = 0;
	fin >> n;
	for(int i = 0; i < n; i++)
		fin >> x[i] >> y[i];
	x[n] = x[0];
	y[n] = y[0];
	for(int i = 0; i < n; i++)
		sol += (x[i] * y[i + 1] - x[i + 1] * y[i]);
	fout << sol / 2.0;
	return 0;
}