A9973. 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.

The first line of input contains integer $n$ ( $1<=n<=10^{5}$ ), 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.