using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;
using System.Text;

public class Program
{

    public void Proc()
    {
        int basstopCount = int.Parse(Reader.ReadLine());

        int[] untin = new int[basstopCount];
        for (int i = 1; i <basstopCount; i++) {
            untin[i] = int.Parse(Reader.ReadLine());
        }

        int curr = 0;
        long ans = 0;
        for (int i = 0; i < basstopCount; i++) {
            int[] inpt = Reader.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();
            ans += curr * untin[i];
            curr -= inpt[0];
            curr += inpt[1];
        }
        Console.WriteLine(ans);
    
    }



    public class Reader
    {
        private static StringReader sr;
        public static bool IsDebug = false;
        public static string ReadLine()
        {
            if (IsDebug)
            {
                if (sr == null)
                {
                    sr = new StringReader(InputText.Trim());
                }
                return sr.ReadLine();
            }
            else
            {
                return Console.ReadLine();
            }
        }
        private static string InputText = @"



4
11
7
3
0 3
2 4
1 0
4 0




";
    }

    public static void Main(string[] args)
    {
#if DEBUG
        Reader.IsDebug = true;
#endif
        Program prg = new Program();
        prg.Proc();
    }
}