#include <iostream>
using namespace std;

int main() {
	int n, a[99], b, c, sum = 0, ans = 0;
	cin >> n;
	for (int i = 0; i < n - 1; ++i) cin >> a[i];
	for (int i = 0; i < n - 1; ++i) {
		cin >> b >> c;
		sum += c - b;
		ans += sum * a[i];
	}
	cout << ans;
}