#include <iostream>
using namespace std;

int N;
long long arie = 0;

#define NMAX 100001

struct pct
{
	int x;
	int y;
}punct[NMAX];

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

	cin >> N;

	for(int i = 0; i < N; ++i)
	{
		cin >> punct[i].x >> punct[i].y;
	}

	punct[N] = punct[0];

	for(int i = 0; i < N; ++i)
	{
		arie += (punct[i].x * punct[i+1].y - punct[i+1].x * punct[i].y);
	}

	cout << arie/2 << endl;
	return 0;
}