#include <iostream>
#include <vector>

using namespace std;

int main() {
	int n;
	cin >> n;

	vector<int> a(n, 0);
	for (int i = 0; i < n - 1; i++) {
		cin >> a[i];
	}

	int paying = 0;
	int people = 0;
	int b, c;
	for (int i = 0; i < n; i++) {
		cin >> b >> c;
		people -= b;
		people += c;
		// printf("%d -> %d: a %d, people %d, pay %d\n", i, i+1, a[i], people, paying);
		paying += a[i] * people;
	}
	cout << paying << endl;
	return 0;
}