|Codeforces Round #326 (Div. 2)|
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.
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.
Print the minimum money needed to keep Duff happy for n days, in one line.
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.