#include "bits/stdc++.h"
#define int long long
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const ll INF = (1LL << 61);
ll mod = 998244353;

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int N; cin >> N;
	vector<int>X(N), Y(N);
	for (int i = 0; i < N; i++)cin >> X[i] >> Y[i];
	int ans = 0;
	for (int i = 0; i < N; i++) {
		ans += X[i] * Y[(i + 1)%N] - X[(i + 1)%N] * Y[i];
	}
	ans = abs(ans);
	cout << ans << endl;
	return 0;
}