#include <bits/stdc++.h>
using namespace std;

const int MAX = 1e5;

struct point {
	long double x, y;
}A[MAX + 3];

int N;
long double ans;

int main() {
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);

	cin >> N;

	for(int i = 1; i <= N; i++) {
		cin >> A[i].x >> A[i].y;
	}

	A[N + 1] = A[1];

	for(int i = 1; i <= N; i++) {
		ans += A[i].x * A[i + 1].y - A[i + 1].x * A[i].y;
	}

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