#include <iostream>
//#include<atcoder/all>
#include <numeric>
#include <cmath>
#include <limits>
#include <stdio.h>
#include <iomanip>
#include <string> 
#include <vector>
#include <algorithm>
#include <utility> 
#include <tuple> 
#include <cstdint> 
#include <cstdio> 
#include <map> 
#include <queue>
#include <set> 
#include <stack> 
#include <deque> 
#include <unordered_map> 
#include <unordered_set>
#include <bitset> 
#include <cctype>
//using namespace atcoder;
using namespace std;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<pair<ll, ll>>>;
vector<bool> seen;
bool C = true;
vector<ll> dist;
ll mod = 998244353;


int main() {
	ll N;
	cin >> N;
	vll X(N), Y(N);
	rep(i, N) {
		cin >> X[i] >> Y[i];
	}
	ll an = 0;
	rep(i, N - 2) {
		an += abs((X[i + 1] - X[0]) * (Y[i + 2] - Y[0]) - (X[i + 2] - X[0]) * (Y[i + 1] - Y[0]));
	}
	cout << an << endl;
}