package net.ipipip0129.yukicoder.no633;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int busStopCount = Integer.parseInt(scan.nextLine());
        int[] busStopMoney = new int[busStopCount - 1];

        for (int i = 0; i < busStopCount - 1; i++) busStopMoney[i] = Integer.parseInt(scan.nextLine());

        int busPassengers = Integer.parseInt(scan.nextLine().split(" ")[1]);
        int toalMonay = 0;

        for (int i = 0; i < busStopCount - 1; i++) {
            toalMonay += busStopMoney[i] * busPassengers;
            String[] data = scan.nextLine().split(" ");
            busPassengers += Integer.parseInt(data[1]) - Integer.parseInt(data[0]);
        }

        System.out.println(toalMonay);
    }
}