#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <sstream>
#include<set>
#include<stack>

//#include<bits/stdc++.h>


using namespace std;



int main() {


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

		if (i) {
			ans += t*a[i - 1];
		}

		t += c;
		t -= b;
		
	}
	cout << ans << endl;


	return 0;




}