#include <bits/stdc++.h>
#include<iostream>
#include<map>
#include<vector>
#include <algorithm>
#include<math.h>
#include <iomanip>
#include<set>
#include <numeric>
#include<string>
using namespace std;


int main()
{
    int n, sum1 = 0;
    cin >> n;
    vector<int> a(n), b(n);
    for (int i = 0; i < n; i++) cin >> a[i] >> b[i];
    for (int i = 1; i < n-1; i++){
        int h, w, p, q, r, s;
        p = a[i] - a[0];
        q = b[i] - b[0];
        r = a[i+1] - a[0];
        s = b[i+1] - b[0];
        sum1 += abs(p*s - q*r);
    }
    cout << sum1 << endl;
}