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

int main()
{
	int N;
	cin >> N;
	vector<int> X( N );
	vector<int> Y( N );
	for( int i = 0; i < N; i++ ) {
		cin >> X[i] >> Y[i];
	}

	int ans = 0;
	for( int i = 1; i < N - 1; i++ ) {
		ans += abs( (X[i] - X[0]) * (Y[i + 1] - Y[0]) - (X[i + 1] - X[0]) * (Y[i] - Y[0]) );
	}

	cout << ans << endl;
}