#include "bits/stdc++.h"
//#include "atcoder/all"
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int N;
	cin >> N;
	vector<long long>X(N);
	vector<long long>Y(N);
	rep(i, N) { cin >> X[i] >> Y[i]; }
	long long ans = 0;
	rep(i, N - 2) {
		ans += abs((X[i + 1] - X[0])*(Y[i + 2] - Y[0]) - (X[i + 2] - X[0])*(Y[i + 1] - Y[0]));
	}
	cout << ans << endl;
	return 0;
}