#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
long a[33], b[33];
int main() {
	long n;
	cin >> n;
	long c, d, ans = 0;
	for (int i = 0; i < n; i++) {
		cin >> a[i] >> b[i];
		if (i == 0) {
			c = a[i], d = b[i];
		}
		a[i] -= c, b[i] -= d;
	}
	for (int i = 1; i < n - 1; i++) {
		ans += abs(a[i + 1] * b[i] - b[i + 1] * a[i]);
	}
	cout << ans << endl;
}