//ALEXANDRU MICLEA

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

using namespace std;
using ll = long long;

#define fast_cin() 	ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)

//VARIABLES



//FUNCTIONS



//MAIN
int main() {

	#ifdef INFOARENA
		freopen("aria.in", "r", stdin);
		freopen("aria.out", "w", stdout);
	#endif

	fast_cin();

	int n; cin >> n;
	vector <pair <double, double>> v(n);

	for (auto& el : v) {
		cin >> el.first >> el.second;
	}

	long double ans = 0;
	for (int i = 0; i < v.size() - 1; i++) {
		ans += v[i].first * v[i + 1].second - v[i].second * v[i + 1].first;
	}
	ans += v[n - 1].first * v[0].second - v[n - 1].second * v[0].first;

	cout << setprecision(5) << fixed << ans / 2;

	return 0;
}