CF588A.Duff and Meat

Duff is addicted to meat! Malek wants to keep her happy for n days. In order to be happy in i-th day, she needs to eat exactly a__i kilograms of meat.

There is a big shop uptown and Malek wants to buy meat for her from there. In i-th day, they sell meat for p__i dollars per kilogram. Malek knows all numbers _a_1, ..., a__n and _p_1, ..., p__n. In each day, he can buy arbitrary amount of meat, also he can keep some meat he has for the future.

Malek is a little tired from cooking meat, so he asked for your help. Help him to minimize the total money he spends to keep Duff happy for n days.

达芙痴迷于肉类！马莱克希望让她在 $n$ 天内都保持开心。为了在第 $i$ 天开心，她恰好需要吃 $a_i$ 千克的肉。

城中有一家大型商店，马莱克打算从那里为达芙购买肉类。在第 $i$ 天，该商店的肉售价为每千克 $p_i$ 美元。马莱克已知所有数值 $a_1, \dots, a_n$ 和 $p_1, \dots, p_n$。每天他都可以购买任意数量的肉，并且可以将已购的肉储存起来供日后使用。

马莱克因烹饪肉类而略感疲惫，因此向你求助。请帮助他最小化为让达芙在 $n$ 天内保持开心所花费的总金额。

The first line of input contains integer n (1 ≤ n ≤ 105), the number of days.

In the next n lines, i-th line contains two integers a__i and p__i (1 ≤ a__i, p__i ≤ 100), the amount of meat Duff needs and the cost of meat in that day.

输入的第一行包含一个整数 $n$（$1 \leq n \leq 10^5$），表示天数。

接下来的 $n$ 行中，第 $i$ 行包含两个整数 $a_i$ 和 $p_i$（$1 \leq a_i,\,p_i \leq 100$），分别表示第 $i$ 天 Duff 所需的肉量以及当天肉的价格。

Print the minimum money needed to keep Duff happy for n days, in one line.

输出使达夫在 n 天内保持开心所需的最少金钱数，结果占一行。

In the first sample case: An optimal way would be to buy 1 kg on the first day, 2 kg on the second day and 3 kg on the third day.

In the second sample case: An optimal way would be to buy 1 kg on the first day and 5 kg (needed meat for the second and third day) on the second day.

在第一个样例中：一种最优方案是第一天购买 1 千克，第二天购买 2 千克，第三天购买 3 千克。

在第二个样例中：一种最优方案是第一天购买 1 千克，第二天购买 5 千克（即满足第二天和第三天所需的肉量）。