using System;
using System.Collections.Generic;
using System.Linq;

class Solution
{
    static void Main()
    {
        var n = int.Parse(Console.ReadLine());
        var a = new int[n];
        var b = new int[n];
        var c = new int[n];
        for (int i = 1; i < n; i++)
        {
            a[i] = int.Parse(Console.ReadLine());
        }
        for (int i = 0; i < n; i++)
        {
            var vals = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
            b[i] = vals[0];
            c[i] = vals[1];
        }

        int pay = 0;
        int customer = 0;
        for (int i = 0; i < n; i++)
        {
            pay += a[i] * customer;
            customer += c[i] - b[i];
        }

        Console.WriteLine(pay);
    }
}