//ALEX ENACHE

#include <vector>
#include <algorithm>
#include <queue>
#include <map>
#include <set>
#include <unordered_map>
#include <time.h>
#include <iomanip>
#include <deque>
#include <math.h>
#include <assert.h>
#include <stack>
#include <bitset>
#include <random>

using namespace std;

#include <fstream>
ifstream cin("input"); ofstream cout("output");
//ifstream cin("aria.in"); ofstream cout("aria.out");

pair < long double, long double > v[100100];

long double triunghi(pair < long double, long double > a, pair < long double, long double > b, pair < long double, long double > c) {
	long double det = a.first * b.second + b.first * c.second + c.first * a.second - a.second * b.first - b.second * c.first - c.second * a.first;
	det = abs(det);
	return det / 2.0;
}

int main() {

	cout << fixed << setprecision(10);

	int n;
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> v[i].first >> v[i].second;
	}

	long double ans = 0;

	for (int i = 1; i < n - 1; i++) {
		ans += triunghi(v[n], v[i], v[i + 1]);
	}

	cout << ans;

	return 0;
}