#include <bits/stdc++.h>
using namespace std;

int main(void){
	int n,a[100];
	cin >> n;
	for(int i=0;i<n-1;i++)
		cin >> a[i];
	int user=0,cost=0;
	for(int i=0;i<n;i++){
		int b,c;
		cin >> b >> c;
		user += c-b;
		cost += a[i]*user;
	}
	cout << cost << endl;
}