#include<iostream>
#include<vector>
#include<string>
#include<sstream>
#include<functional>
#include<algorithm>
#include<map>
#include<queue>
#include<stack>
using namespace std;
int main() {
	int n;
	cin >> n;
	vector<int> x(n), y(n);
    cin >> x[0] >> y[0];
	for (int i = 1; i < n; i++) {
		cin >> x[i] >> y[i];
		x[i] -= x[0];
		y[i] -= y[0];
	}
	int ans = 0;
	for (int i = 2; i < n; i++) {
		ans += abs(x[i] * y[i - 1] - x[i - 1] * y[i]);
	}
	cout << ans << endl;
}